Step 2

Make the custard icing. Beat the 100g softened butter with the cream and custard powder for about 2 minutes, until pale, light and fluffy. Add the powdered sugar a little at a time, beating until smooth and fully incorporated. Spread evenly over the chilled base. Chill for at least 10 minutes, until firm to the touch and no longer soft.

Step 3

Melt the dark chocolate with the 50g butter in the microwave in short bursts, stirring between each, until just smooth. Spread over the chilled custard layer. Refrigerate for about 2 hours, until the chocolate has set completely.

Step 4

Lift the slab out of the tin using the lining paper and cut into 12 squares. A warm, dry knife gives the cleanest edges.