1
Prepare the Tin. Butter a 20 cm square tin and line it with baking parchment, leaving the paper hanging over two opposite sides so you can lift the slab out to cut it.
2
Melt Basic Ingredients. In a large saucepan, gently melt the butter, sugar, syrup and cocoa over low heat until smooth and glossy. Do not let it bubble, or it turns greasy and splits later.
3
Mix in Biscuits and Additions. Take the pan off the heat and stir in the broken digestive biscuits until every piece is coated. Break them in a bag rather than blitzing them, and leave plenty of big chunky pieces, which stop it setting into a dense slab.
4
Melt the Chocolate. In a separate heatproof bowl, melt the milk and dark chocolate over barely simmering water, or in the microwave in short bursts, stirring between each one. Keep water away from it, or it will seize.
5
Combine Mixtures. Stir half of the melted chocolate into the biscuit mixture until it is all coated. This is what makes the base set firm rather than crumbly.
6
Transfer to Tin. Pour the mixture into the prepared tin and press it down hard, working from the middle out to the corners. Air pockets left in now are where the squares crumble.
7
Top with Remaining Chocolate. Spread the remaining melted chocolate over the top and level it. Stir a few drops of flavourless oil into it first if you like, so the top cuts cleanly instead of cracking.
8
Add a Salty Finish. Optionally, scatter a pinch of flaky sea salt over the chocolate now, while it is still soft, because it will not stick once it has set.
9
Chill and Set. Chill in the fridge for at least 2 hours. Overnight is better, because a fully cold slab gives clean-edged squares.
10
Cut and Serve. Take it out and let it stand for 10 to 15 minutes, since cutting it fridge-cold is why the top cracks. Lift it out by the parchment and cut straight down with a long sharp knife warmed in hot water and dried, wiping between cuts. Dust with icing sugar.