Preheat the oven to 200C/180C Fan Fan/Gas 6. To make the Bundt cake, in a blender or large bowl, combine the butter and powdered sugar until light and fluffy. Add one egg at a time with a tablespoon of flour. Mix until the egg is just incorporated into the mixture. Using a metal spoon, stir in the remaining flour a third at a time and finally incorporate the vanilla and milk.
Transfer the mixture to the cake tin and bake for 45 minutes. Stick a skewer in the middle of the cake to check that it is well cooked. Use foil to cover the top if it needs a little more time and to prevent it from browning too much on top. Leave to cool to room temperature then unmold and set aside.
To make the sponge cake, follow the same steps as for the bundt cake but using the measurements of the sponge cake. Grease a 23cm springform pan and line with parchment paper. Pour the mixture into the mold and bake for 30 minutes. Stick a skewer in the middle of the cake to check that it is well cooked. Use foil to cover the top if it needs a little more time and to prevent it from browning too much on top. Leave to cool to room temperature then unmold and set aside.
To make the jam, cut the strawberries into quarters and put them in a bowl. Cover with the Dubonnet and leave to soak for 10 minutes.
Strain the fruit, reserving the Dubonnet and add them to a saucepan with the raspberries, caster sugar and 2 tablespoons of the reserved Dubonnet. Increase the heat to medium-high until the mixture comes to a complete boil. Reduce heat to medium, stirring constantly, and simmer for about 25 minutes or until jam has thickened. Remove from heat and let cool.
To make the buttercream, use an electric mixer or a whisk to beat the butter until fluffy. Add a third of the icing sugar and continue beating until well incorporated. Add the vanilla extract, milk and remaining icing sugar and mix until the buttercream is light and fluffy. It will take about 3 minutes.
To assemble, cut the sponge cake in half and spread all but 3 tablespoons of jam on the bottom half of the sponge cake. Using a piping bag, pipe two-thirds of the buttercream onto the jam using a large star-shaped nozzle, then place the other half of the sponge cake on top.
Cut the base of the Bundt cake to make sure it is flat. Place the rest of the jam in the center of the top of the sponge cake and place the Bundt cake on top. Using the small star-shaped nozzle, pipe the reserved buttercream decoratively around the exposed sponge and place the halved cherries on top to cover the seam between the sponge and the bundt . Use the remaining buttercream to glue the raspberries and blueberries in place – one raspberry at the very top of each bundt peak and the blueberries around the outside. Sprinkle with icing sugar.