Checkerboard cake

Prepare the cake batter according to the instructions on the packet. Butter two 29 x 26 x 10 cm loaf tins or spray with non-stick spray. Line the cake tins with baking paper. Butter lightly once more or spray with non-stick spray.
Turn the chocolate cake batter into one of the loaf tins and the vanilla cake batter into the other. Bake for 30 to 35 minutes until done. Cool slightly in the loaf tins before turning out onto cooling racks to cool completely.
Trim the top of each loaf evenly, ensuring the two loaves are the same size. Cut each loaf horizontally into three layers, and then vertically into three sections to form long cake fingers.
Line one of the loaf tins with plastic wrap.
Spread the sides of one chocolate cake finger with apricot jam and place in the centre of the loaf tin.
Place two vanilla cake fingers on either side of the chocolate cake finger, pressing together gently. Spread the tops of the cake fingers with apricot jam.
Repeat this process using the remaining cake fingers, alternating the chocolate and vanilla cake fingers to obtain a checkerboard effect. Repeat until there are four layers, cover the cake with plastic wrap and chill until firm.
Slice with a sharp knife just before serving. Prepare the instant custard with boiling water as indicated on the back of the packet and flavour half the custard with grated chocolate to make a chocolate custard.
Serve with the checkerboard cake slices.