Nanaimo Bars

Cook the chocolate base. Set a heatproof bowl over a pan of barely simmering water and whisk the butter, sugar, cocoa and beaten egg constantly for 3-5 minutes, until the mixture turns glossy, thickens and reaches 71°C on a thermometer. Cooking the egg to this temperature over gentle heat makes it safe to eat without scrambling it. Keep whisking the whole time, since the egg can curdle if it sits still over the heat.

Finish the base. Off the heat, stir in the biscuit crumbs, coconut and chopped nuts until the mixture looks like damp, clumping sand. Press it very firmly into the lined 20 cm metal tin, pushing right into the corners, since a loosely packed base will crumble when you cut the bars later. A flat-bottomed glass or the back of a spoon makes this easier.

Chill the base. Chill the tin for 20 minutes, until the base feels firm to the touch. This brief chill sets the butter in the base enough to support the custard layer without it sinking in. A metal tin conducts the cold efficiently, speeding this step up compared with glass.

Make the custard layer. Beat the soft butter, custard powder and cream together until smooth, then beat in the icing sugar and vanilla for 2-3 minutes until light and spreadable. The custard powder is what gives this layer its pale yellow colour and familiar flavour, without needing to cook anything. Spread it evenly over the chilled base right to the edges.

Chill the custard layer. Chill for 30 minutes, until the custard layer feels fully set and no longer soft when touched. Letting it firm up properly now stops it from melting into the chocolate layer poured on top. A fully set custard layer also gives a cleaner, more distinct line when the bars are sliced.

Make the chocolate top. Melt the chocolate and butter together over the same barely simmering water until smooth, then let it cool for 5 minutes before spreading it thinly over the custard layer. Cooling the chocolate slightly first matters, since pouring it on too hot can melt the custard layer underneath. Spread it right to the edges in a thin, even layer.

Chill and cut. Chill for 20-30 minutes, until the chocolate is just set but not fridge-hard, then cut into 16 bars using a hot, wiped knife. Cutting at this just-set stage is the trick: fully hardened chocolate cracks and splinters instead of slicing cleanly. Wipe and rewarm the knife between cuts for the neatest edges.