Preheat the oven to 180°C (350°F). Grease and line a 7-inch cake pan with parchment paper.
Combine dry ingredients in a large mixing bowl: add the plain flour, baking powder, custard powder, and castor sugar.
Add the wet ingredients: softened butter, eggs, vanilla essence, and milk into the bowl with the dry ingredients.
Mix until smooth: Using a hand whisk or electric mixer, combine everything until the batter is smooth and well mixed.
Pour the batter into the prepared pan and smooth the top.
Bake for 50-60 minutes, or until a skewer inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Dust with icing sugar before serving.
