Preheat oven to 160°C (140°C fan-forced). Grease deep 22cm-round cake pan; line base with baking paper.

Make sponge cake according to the directions on packet; pour mixture into pan. Bake about 35 minutes. Turn cake onto wire rack to cool.

To make custard filling; combine cornflour, custard powder and sugar in medium saucepan. Gradually add combined milk, cream and extract; stir over low heat until mixture boils and thickens. Add butter; simmer, stirring, 3 minutes. Remove pan from heat; stir in egg yolk. Place custard in large bowl; cover with plastic wrap. Cool. Divide custard mixture between two bowls. Stir melted chocolate into one bowl. Leave remaining custard plain.

Combine cold coffee, liqueur, brandy and sugar in small jug; mix well. Split cold cake into four layers. Place first layer on serving plate; brush well with coffee mixture.

Spread half the plain custard over cake. Top custard with second layer of cake; brush with coffee mixture. Spread a third of the chocolate custard over cake. Place third layer of cake on top of custard; brush with coffee mixture then spread with remaining plain custard. Top with fourth layer of cake; brush with coffee mixture.

Using a large spatula, spread remaining chocolate custard over top and side of cake; refrigerate 3 hours or overnight.