Preheat the oven to 180ºC / 160ºC Fan / Gas 4 / 350ºF. Prepare the baking tins by spraying them with cake release or line them with greaseproof paper.
In a bowl, cream the butter and sugar together using an electric handheld whisk. Mix until pale and smooth.
Add the eggs to the creamed butter one at a time, beating well after each addition. Add the flour and baking powder to the bowl and beat slowly, mixing until no white streaks of flour are visible. Take care not to overmix.
Add the vanilla extract and milk and fold into the mixture using a rubber spatula or wooden spoon.
Divide the mixture evenly between the tins and bake for 25 minutes until golden brown. A skewer inserted into the centre of the cakes should come out clean. Remove from the oven and leave the cakes in the tins to cool.
For the strawberry buttercream
Add the room temperature butter to a stand mixer (or use a large bowl and handheld electric whisk) and beat for a couple of minutes until pale yellow and creamy. Next, add in the icing sugar half a cup at a time.
Once you’ve added all the icing sugar, pour in the vanilla, milk and strawberry jam and continue to beat until it’s smooth and creamy. You can add a small splash of pink food colouring if you want to intensify the pink colour of the icing.
Assembly
Place one of the sponges on a cake board or serving plate. Use a 5cm cookie cutter to cut a hole from the centre of the sponge. You can now use this sponge as a template for the others. Place the next sponge on top of the sponge with the hole and push the cookie cutter through. Repeat for all sponges. Reserve one of the cut out pieces of cake for later.
Spread all the sponges with a layer of strawberry buttercream and stack them on top of each other. Add ¾ of a bag of HARIBO Starmix into the hole in the centre of the sponges. Plug the hole in the top with the leftover circle of cake that you saved earlier.
Spread the remaining strawberry buttercream on top of the cake and down the sides using a palette knife to smooth.
To decorate
Whip the double cream, icing sugar and pink food colouring until it reaches stiff peaks. Transfer the cream to a piping bag fitted with a Wilton 1M nozzle. Pipe swirls on top of the cake. Decorate the top of the Piñata cake with a handful of HARIBO Starmix and place a sweet on top of each cream swirl.