Step 1

Preheat oven to 180°C/160°C fan forced. Grease a 5cm-deep, 20 x 26cm rectangle cake pan. Line with baking paper, allowing paper to overhang sides.

Step 2

Arrange biscuits in a single layer over the base of the cake pan. Pour over condensed milk. Spoon over lemon curd then sour cream, spreading slightly to smooth the surface. Sprinkle over cake mix, spreading with a fork. Arrange butter in rows over the cake mix and pour over milk.

Step 3

Bake for 35-40 minutes. Set aside, in the pan, until cooled completely. Use the baking paper to transfer the cake to a serving board. Dust with icing sugar, if using, to serve.