Preheat the oven to 180ºC, fan 160º, gas 4. Grease and line 3 x 15cm sandwich cake tins with baking paper. In a large bowl beat the butter, caster sugar, vanilla extract, eggs and flour together with an electric hand mixer until light and smooth.
Divide 2/3 of the mixture between 2 tins. Add a few drops of red food colouring to the remaining mixture and stir to combine. Spoon the remaining mixture into the third cake tin. Bake the cakes for 35-40 minutes or until a skewer comes out clean. Turn out onto a cooling rack and leave to cool.
Trim off the tops of each sponge using a bread knife to create a flat, level surface. Put one of the plain sponges onto a plate and cover the top with strawberry jam. Repeat with the pink sponge in the middle, then top with the third sponge.
To make the icing, whisk together the butter and icing sugar. Place ¼ of the icing mix into a separate bowl and stir in the green food colouring. Add the red food colouring to the remaining icing.
Spread the pink icing over the top and sides of the cake using a palette knife or a normal kitchen knife. Spoon the green icing into a piping bag, then pipe green stalks of the mixture up the sides of the cake. Arrange the flower decorations on top of each stalk, then top the cake with the Happy Birthday candles. Decorate with the butterfly decorations before serving.