1 Preheat the oven to 180c, 160c fan, gas 4. Grease and line a 28cm x 18cm traybake tin, ensure the baking paper comes up at least 2cm above the sides of the tin.
2 Put the margarine and caster sugar in a large mixing bowl and mix with an electric hand-whisk until light and fluffy. The sugar will provide a hint of added sweetness. Add the eggs one at a time whisking well between each addition. Sift over the flour and fold in the 2 tbsp ofcustard until combined.
3 Set aside 8 custard creams, put the remainder in a bowl and break up into small pieces with your fingertips. Fold the broken pieces through the batter. Spoon the batter into the prepared tin and level the surface.
4 Lay the reserved custard creams on top of the batter, lightly pressing down so the bases are nestled into the batter for some crunch. Bake for 45-50 mins until an inserted skewer comes out clean – after 20 mins, working quickly, cover loosely with a large piece of foil to prevent the biscuits from browning too much, cover without pulling the cake completely out of the oven (scrunch the corners of the foil, so the foil is slightly domed, this will ensure there is enough room for the cake to rise further).
5 Allow to cool in the tin for 10 mins, then use the baking paper on the longer sides to carefully lift out onto a work surface. Use a cake mover to move the cake to a wire rack whilst at the same time removing the baking paper. Leave to cool completely.
6 To decorate, melt the chocolate in a bowl set over a pan of simmering water. Once melted remove from the hob but keep the bowl over the hot water whilst you make up the remaining custard.
7 Heat the custard to just before boiling point to give the dish somewarmth. Working quickly while the custard is still hot, remove the bowl of melted chocolate from the saucepan, add 2 tbsp of the hot custard and stir quickly until combined and smooth. Immediately drizzle of the traybake for addedcreaminess. Set aside for 1 hour to set. Cut into 15 pieces. Cover the surface of the leftover custard with clingfilm. When ready to serve the traybake, serve with the leftover custard at room temperature or warmed, as liked.