Step 1

Preheat oven to 160C. Line a 27cm x 18cm slice tin with baking paper, leaving plenty hanging over to make it easy to remove the slice.

Step 2

Place the biscuits in a large ziplock bag and bash with a rolling pin to form a fine crumb.

Step 3

Place in a bowl with melted butter and mix to combine. Press into the base of the prepared slice tin. Bake for 8 minutes.

Step 4

Meanwhile, in the bowl of a stand mixer or with handheld beaters, whisk egg whites into soft peaks. Remove to a separate bowl and set aside.

Step 5

Beat egg yolks and sugar in the stand mixer until pale. Add vanilla and butter and continue beating until well combined. Beat in milk, then flour.

Step 6

Add the whipped egg whites and fold in with a metal spoon until just combined. Pour over biscuit base in slice tin and bake for 30 minutes.

Step 7

Allow to cool. Remove from tin and slice into squares. Dust with icing sugar and serve.