1.Preheat oven to 150°C/300°F. Grease deep 15cm (6-inch) round cake pan; line base and side with baking paper, extending paper 5cm above side. Make cake according to directions on packet. Spread mixture into pan until three-quarters full; bake about 1 hour.
2.Stand cake in pan 5 minutes before turning, top-side up, onto wire rack to cool. Level cake top; turn cake, cut-side down, onto 20cm (8-inch) round prepared cake board or cake stand. Brush cake lightly with jam.
3.On surface dusted with icing sugar, knead white icing until icing loses its stickiness; tint icing yellow using food colouring. Roll icing into a circle large enough to cover cake. Using rolling pin, carefully lift icing over cake.
4.Using sugared hands, smooth icing over cake. Trim neatly around base. Cut out lengths of little girl shapes from paper. Position little girls around cake; secure with tape.
Use any remaining cake mixture to make some extra cupcakes for the party. The cake can be completed at least two days ahead. White icing is also known as fondant (it’s the traditional icing used on wedding cakes). It is available in packets from the supermarket and cake decorating shops.