Pre heat oven to 180°C/160°FF
Line a square 23cm x 23cm cake pan with baking paper
In the bowl of a stand mixer, beat butter and sugar on medium-high speed for 5 mins
Gradually add the egg, stopping the mixer and scraping the sides once or twice
Mix through the vanilla
Add flour, alternating with the milk (⅓ of the flour, ½ of the milk etc.). Mix until just combined
Scoop mixture into prepared pan, smooth over and bake for 25 minutes, until lightly golden and a cake tester comes out clean.
Once cooked, cool in pan for 10 minutes, then turn out onto a cake cooler to cool completely.
Once completely cool, cut into 16 squares, then wrap each square in glad wrap and place in the freezer until frozen (frozen cake squares are much easier to handle and coat!).