You will want to build this biscuit cake while the chocolate custard is still piping hot. That trapped heat works like magic to soften up the plain biscuits, and a good chill in the fridge transforms the layers into a sliceable, tender cake without ever turning on the oven.
Make sure you whisk the flour, cornstarch, egg yolk, and cocoa right into cold milk before you ever turn on the burner. Cook it down until nice and thick, keeping it moving so you don't get any lumps, then stir in the cream as called for. Layer that warm custard with the biscuits and let it chill until firm - overnight is honestly best. If you let the custard cool down before building your layers, the biscuits will end up way too crunchy.
