Preheat oven to 180C/350F (or fan forced 160C/320F).
Grease a 20 cm x 30 cm / 8" x 12" cake pan. Line with baking paper (parchment paper), leaving an overhang on all sides (to make it easy to remove).
In a large mixing bowl, whisk together flour and baking bowder.
In a separate mixing bowl, combine butter, vanilla and sugar and beat with an electric hand mixer until pale and fluffy.
Add eggs 1 at a time, beating well after each addition so the batter is smooth, not curdled.
Add half the flour and gently fold to combine, then stir in half the milk. Repeat with remaining flour and milk.
Pour the batter into the prepared pan and bake for 25 minutes until a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for 10 minutes, then turn out onto a wire rack (leave the baking paper attached).
Cut cake in half horizontally, then each half into 15 squares (5 x 3), or 18 rectangles. Freeze for 1 to 1.5 hours or leave it in the fridge overnight