Chocolate has one way of going wrong
Everything else in this recipe is stirring. The melting pan is the only place it can fail, and it fails in exactly one way: the chocolate gets too hot and goes grainy.
Chocolate is an emulsion. Cocoa solids and sugar are suspended in cocoa butter, and that suspension holds together over a fairly narrow temperature range. Push past it, which for milk chocolate is around 45°C and not very hot at all, and the emulsion breaks. The fat separates out, the solids clump, and what was glossy and fluid goes thick, dull and gritty.
There is no fixing it. So: the lowest heat your hob will hold, constant stirring, and take the pan off while a few unmelted pieces are still visible. The residual heat in the pan finishes them, and stopping early is the only reliable insurance.
The butter and syrup in with the chocolate help, because they give the mixture more tolerance than chocolate alone would have.
Marshmallows go in last, off the heat
A marshmallow is sugar and air in a gelatine foam, and gelatine melts at around body temperature. Drop them into hot chocolate and they do not sit in it, they dissolve into it.
What you lose is the entire point. The white bits are the contrast, in colour and in texture, and without them you have a chocolate biscuit slab, which is a perfectly good thing and is not rocky road.
Five minutes off the heat before they go in is enough. The mixture should be warm rather than hot, and thick rather than pourable. Stir them through with a few turns and get it into the tin: the longer they sit in warm chocolate the more they slump.
The syrup, again
It is doing the same job here that it does in the flapjacks on the next page, and it is worth knowing once because it explains both.
Set chocolate on its own is brittle. That is a virtue in a bar, where the snap is half the pleasure. It is a serious problem in a slab studded with hard biscuit that somebody has to cut into squares, because a brittle matrix with hard lumps in it cracks along whatever line the lumps suggest.
Golden syrup is an invert sugar. Its molecules come in two shapes that cannot stack neatly into crystals, so it stays soft and sticky and keeps the whole set mixture slightly pliable. Three tablespoons is enough to cut cleanly and to bite without cracking a filling, and not enough to taste as a separate sweetness.
Stop bashing while there are still chunks
Biscuits in a bag with a rolling pin is the best job in the recipe and it is also the one most likely to be overdone, because it is satisfying and there is no obvious point to stop.
You want a mix: some pieces the size of a thumbnail, some smaller, and some dust. Dust alone packs into a dense brick with no texture at all, and it absorbs a surprising amount of the chocolate, so the slab ends up dry. The chunks are the thing you bite into.
Push the air out of the bag and seal it properly before the rolling pin arrives. A bag with air in it bursts, and the resulting distribution of biscuit across a kitchen is memorable.
Why it is the best recipe here for a small child
No oven at all, which removes the single biggest hazard and most of the waiting. No raw egg, so nothing needs washing off hands halfway through. No knives in the main method. And the two most enjoyable jobs in the whole recipe, smashing the biscuits and stirring everything together, are entirely theirs.
That leaves the melting pan, which is supervised because melted sugar and chocolate are hot and stick to skin, and the cutting at the end, which wants an adult and a big knife. Everything in between can be handed over completely.
Cut the squares smaller than looks right. It is extremely rich and a piece the size of a matchbox is a portion.
Common questions
Why did my chocolate go grainy?
It got too hot, and there is no reversing it. Chocolate is cocoa solids and sugar suspended in cocoa butter, and past about 45°C for milk chocolate the emulsion breaks: the fat separates and the solids clump into a thick, dull, gritty mass. Melt it over the lowest heat you have, stir constantly, and take it off while a few unmelted pieces remain. They finish in the residual heat.
When do you add marshmallows to rocky road?
Last, and only once the chocolate has cooled to warm rather than hot. Marshmallow is mostly sugar and air held in a gelatine foam, and hot chocolate melts it straight into the mixture. The white bits disappear, the texture goes uniform, and what you have made is a chocolate biscuit slab. Five minutes off the heat before they go in is enough.
Why is there syrup in rocky road?
For the same structural reason it is in flapjacks. Set chocolate on its own is brittle: it snaps cleanly, which is what you want in a bar and not what you want in a slab full of hard biscuit that has to be cut into squares. Golden syrup is an invert sugar that resists crystallising, so it keeps the set mixture pliable enough to cut without shattering and to bite without cracking a filling.
How do you cut rocky road without it shattering?
Use a large knife and press straight down in one movement rather than sawing. A sawing action puts sideways force into a brittle slab and it cracks along its own lines. If it is very firm, let it sit out of the fridge for ten minutes first. Warming the blade under a hot tap and drying it also helps, which is the same trick as for cheesecake.
Can you make rocky road without marshmallows?
Yes, and there are two reasons people need to. Most marshmallows contain gelatine, which comes from pork or beef, so they are not vegetarian unless the packet says so. And some people simply dislike them. Extra raisins, glace cherries, chopped dried apricot or broken honeycomb all fill the same gap, which is something soft and sweet against the hard biscuit.
What biscuits are best for rocky road?
Digestives, which are sturdy enough to stay in recognisable chunks and plain enough not to fight the chocolate. Graham crackers are the American equivalent and noticeably sweeter, so drop the syrup to two tablespoons if you use them. Rich tea work and go softer. Shortbread makes a richer and much more expensive version. Anything with a cream filling turns the whole thing greasy.
How long does rocky road need to set?
Three hours in the fridge as a minimum and overnight is better. What is setting is the cocoa butter, which is a fat with a fairly wide melting range, so it goes from soft to firm gradually rather than at a single point. Rushing it into the freezer sets the outside fast and leaves a soft middle, and it can make the chocolate bloom white on the surface.
Is rocky road good for cooking with children?
It is close to the best thing in this section for it. There is no oven at all, no raw egg, no knife work in the main recipe, and the two most enjoyable jobs, bashing biscuits in a bag and stirring everything together, are both entirely theirs. The only supervised step is the melting pan, and the only adult one is cutting the finished slab.