Preheat the oven to 180C/350F/Gas 4. Grease and line a Swiss roll tin.
Cream the butter and sugar together in a bowl until light and fluffy. Add the eggs and milk and beat for five minutes.
Add the flour and baking powder and mix until well combined. Fold in the raspberries, passion fruit and food colouring.
Spoon the mixture into the Swiss roll tin and cook for 25-30 minutes, or until golden-brown and the top is springy to the touch.
For the frosting, beat the butter and icing sugar together for five minutes, or until pale and fluffy. Add the food colouring, vanilla and lavender and mix until well combined.
Spoon the frosting into a piping bag fitted with a star-shaped nozzle.
To assemble the pop-up cakes, cut out circles of cake using the pop-up tube as a cutter. Layer the sponge circles, frosting, jam and fruit inside the pop up tube. Finish with a layer of frosting and decorate with hundreds and thousands.