The syrup is holding it together
Three ingredients do the binding here and only one of them is obvious. Butter is fat, oats are the substance, and the golden syrup is the thing most people read as sweetness and which is actually the structure.
Ordinary sugar crystallises as it cools. Its molecules are all the same shape, they stack neatly, and what you get is a hard brittle solid. That is excellent in honeycomb and fatal in a flapjack, which needs to be cut into squares and bitten without cracking anything.
Golden syrup is an invert sugar: the molecules have been split into two different forms that get in each other's way and cannot line up into crystals easily. So it stays soft and tacky instead of setting hard, and it keeps the whole slab pliable. Honey and maple syrup do the same job for the same reason. Replacing the syrup with more granulated sugar does not.
Ten minutes, and then cut
This follows directly from the syrup, and it is the single most common way flapjacks go wrong after they have come out of the oven perfectly well.
Hot, the syrup is still completely fluid. A knife goes through and the two halves flow gently back together behind it, so you end up cutting the same slab repeatedly and achieving nothing.
Cold, the syrup has set and the slab is one solid piece with hard oats suspended in it. The knife does not cut it so much as break it, and it breaks wherever the internal stresses say rather than where you put the blade. That is the shattered tray of rubble everybody has produced at least once.
Ten minutes out of the oven is the point where it is set enough to hold an edge and soft enough to take one. Cut the whole grid then, and leave the cut pieces in the tin to go completely cold before you lift anything out.
They look underdone when they are done
A flapjack sets as it cools, not in the oven. Pull the tin out when the top is golden, the edges are a shade darker and the middle still looks soft and faintly wobbly. It will firm up on the side over the next twenty minutes.
If the middle looks properly cooked while it is still in the oven, it has gone too far, and you will find out about it an hour later when you try to eat one. This is the same instruction as for a brownie and it is disbelieved for the same reason: everything else we bake is done when it looks done.
Five minutes either side of the stated time is the dial between chewy and crunchy. Twenty minutes gives something soft and sticky, thirty gives a proper snap. Neither is wrong and it is worth baking two batches to find out which one your household actually wants.
Press harder than feels necessary
The other structural failure, and it happens before the tin ever reaches the oven. Oats bind to each other through the syrup, and syrup can only bind oats that are touching. A mixture spooned in and levelled off has air between the oats, and air does not hold anything together.
Press it down hard with the back of a spoon until the surface is flat and looks compacted, and pay particular attention to the corners. Corners are where a flapjack always breaks first, because they are the part people press least and the part that gets the most oven heat.
The honest answer about making them healthy
This question gets asked a great deal and it deserves a straight answer rather than a substitution list. A flapjack is butter, sugar and syrup with oats in it. The butter and the syrup are not an indulgence bolted onto an oat bar, they are the reason it holds together at all.
What genuinely works: cut the sugar by about a third before the texture starts to suffer, swap a quarter of the oats for seeds, and cut them smaller. What does not work is removing the fat or replacing the syrup with fruit puree, which gives you a damp oat cake that will not keep and that nobody in the house will eat twice.
It is a good thing in a lunchbox because it is dense, it keeps for a week, and it costs pennies. That is a fair description without needing it to be something else.
Common questions
Why do my flapjacks fall apart?
They were not pressed hard enough into the tin, or they were cut cold. Pressing is what puts the oats in contact with each other so the syrup can bind them, and a loosely spooned mixture bakes into something crumbly however good the recipe. Press with the back of a spoon until the surface is flat and compacted, and get right into the corners, which is where they always break first.
When should you cut flapjacks?
Ten minutes out of the oven, and the window is genuinely that narrow. Cut them hot and the slab has not set, so the pieces slump back together. Cut them cold and the syrup has set solid, so the knife shatters the slab along its own lines rather than yours. At ten minutes it is firm enough to hold a cut and soft enough to take one. Then leave the cut pieces in the tin until completely cold.
Why are my flapjacks hard?
Overbaked, or too much ordinary sugar and not enough syrup. Flapjacks look underdone when they are ready: the edges are golden and the middle still looks soft and slightly wobbly, and it sets as it cools. If the middle looks done in the oven, they will be hard on the plate. Five minutes is the difference between chewy and crunchy, so it is worth finding your own number.
Can you use honey instead of golden syrup?
Yes, and it changes the texture a little. Honey is thinner than golden syrup and has a stronger flavour, so the mixture is looser and the flapjacks come out chewier and taste of honey. Maple syrup does the same. What matters is that all three are invert sugars that resist crystallising, which is the structural job being done. Swapping the syrup for more ordinary sugar does not work.
What oats do you use for flapjacks?
Rolled oats, sometimes sold as porridge oats or old-fashioned oats. Jumbo oats give a chunkier, looser flapjack that breaks more easily. Instant or quick oats are cut smaller and absorb more, so they make a denser, more uniform bar that some people prefer and that is noticeably less oaty. Steel cut or pinhead oats do not work at all: they never soften in the time available.
How do you make flapjacks healthier?
Honestly, you mostly do not, and it is worth being straight about that. A flapjack is butter, sugar and syrup held together by oats, and the butter and sugar are the structure rather than an indulgence bolted on. You can cut the sugar by a third before the texture suffers, swap some oats for seeds, and reduce the portion size. What you cannot do is remove the fat and the syrup and still have a flapjack.
Why weigh syrup in the pan?
Because a spoon covered in golden syrup is a spoon you then have to deal with, and about a third of what you measured stays on it. Put the pan on the scales, zero it, and pour the syrup straight in. It is faster, more accurate and there is nothing to wash. The same trick works for honey, treacle and peanut butter.
Can children make flapjacks?
Most of it, and it is one of the better first recipes because there is no raw egg, no knives for the main work and nothing that needs judgement. Lining the tin, weighing, stirring in the oats and pressing the mixture down are all theirs. The melting pan is supervised, because melted sugar is far hotter than boiling water and it sticks to skin, and the cutting is a job for an adult.