Preparation. Preheat oven to 180°C/350°F. Grease with butter the base and sides of a 23cm (9-inch) square baking pan. Line with parchment paper.
Cream the butter and sugar together. In a bowl of a stand mixer fitted with a paddle attachment, cream the butter and sugar on medium for 3 minutes until pale and creamy.
Add the eggs. Add the eggs, one at a time, beating between each addition until combined. If they start to curdle, add a tablespoon or two of flour and the mixture will come back together again. Scrap down the base and sides of the bowl.
Add in the dry ingredients and milk. In a separate bowl, sift in the flour, baking powder and add in the salt. Whisk to combine. Add into the batter, half the flour mixture. Beat until combined. Add the milk and beat until combined, scraping the base and sides of the bowl. Add the remaining flour and mix until a few flour streaks remain.
Bake the sponge. Spoon the batter into the prepared baking pan and place in the oven. Bake for 40-45 mins until a skewer comes out clean. The cake should spring back when lightly touched in the centre. Remove from the oven and allow to cool in the tin for ten minutes before turning out onto a wire rack to cool completely.
Cut the sponge and freeze. Once the cake is cool, use a serrated bread knife to trim the outside edges off the cake then cut the sponge into 16 squares. Freeze for 30 to 40 minutes - until the sponge is very cold but not frozen through.
Make the chocolate glaze. To make the icing, mix all the ingredients together in a bowl and stir until the butter is melted and the mixture is smooth.