Beat together the butter, sugar, eggs, flour and baking powder in a bowl until pale and creamy. Stir in a little milk so that the mixture drops easily from a wooden spoon when tapped against the side of the bowl.
Divide the mixture evenly between 2 greased, baselined and floured 18 cm (7 inch) sandwich tins (see
Make the buttercream. Beat together the butter and icing sugar in a bowl until pale and creamy. Add the vanilla, pink food colouring and measurement water and beat well. Sandwich the cakes together using the jam and a quarter of the buttercream. Spread the remaining icing over the top and side of the cake. Scatter with the sweets and sugar sprinkles. Add birthday candles, if liked.