Ingredients:  
19g (2 heaped tbsp) custard powder
1 tbsp caster sugar
250ml/9fl oz milk

1 egg
250ml/9 fl oz caster sugar
250ml/9 fl oz milk
125ml/4½ fl oz oil
500ml/17½ fl oz plain flour
4 tsp baking powder
4 tbsp cocoa powder

50g/2oz melted butter
6 tbsp icing sugar
1 tbsp cocoa powder
1 tbsp milk

Method:
1. Preheat the oven to 180°C and lightly grease a large baking dish.
2. Prepare the custard by putting 2/3 of the milk in a pan with 1 tbsp sugar and bringing to boil. Mix the custard powder with the remaining milk and stir into the hot milk until the custard thickens. Set aside.
3. Put the egg and sugar into a bowl and mix until pale. Add milk, oil, flour, baking powder and cocoa and mix until smooth. Pour the mixture into the prepared dish and spoon the custard on top. Bake for 30-40 mins or until a skewer inserted in the middle of the cake comes out clean.
4. For the icing mix together melted butter, icing sugar, cocoa and milk in a small pan and stir on a low heat until smooth. Drizzle over the cake, leave to cool and cut into squares.