Preheat the oven to 180ºC. Cream the butter and caster sugar together until pale, light and fluffy. Gradually add the eggs, mixing well between each addition. Sift in the flour and mix to combine. Add the milk and vanilla, and mix again until smooth.
Spoon the mixture into the tin and level with a palette knife.
Bake on the middle shelf of the preheated oven for 60–70 minutes until well risen, golden and springy to the touch, or when a skewer inserted into the middle of the cake comes out clean. Leave to cool in the tin.
Make the topping by whipping the butter, icing sugar and 2 tbsp water together until light and fluffy. Use a palette knife to spread the buttercream over the cooled cake. Pile on a mixture of the sweets and slice into small squares.