Preheat the oven to 190°C/gas mark 5. Grease and line three 18cm loose bottomed sandwich cake tins.
In a stand mixer, add the sugar and butter and beat on a medium speed for 4-5 minutes until light and fluffy. Add the beaten eggs, bit by bit, beating well after each addition and adding a little flour if it starts to curdle.
Once all the eggs are incorporated, add the flour, baking powder and milk and beat briefly until just combined. Finish off with a flexible spatula if need be to avoid over-beating in the mixer.
Divide the mixture between the tins, gently smoothing the surface so they are even. Bake for 20 minutes or until golden and the cake springs back when lightly pressed with a finger. Turn out on to a cooling rack and leave to cool completely.
To make your frosting, beat the butter until smooth. Very gradually add the sugar, bit by bit, beating well after each addition. Add the vanilla extract and salt and then the cream, a spoon at a time. Beat for another 1-2 minutes until voluminous and fluffy.
Fit a piping bag with a 1cm round tip and fill the bag with buttercream. If the cakes aren’t even, carefully use a serrated knife to slice away any cake on top to make it level. Set aside the cakes once they are cut.
Place one of the cake layers on to a cake stand. Carefully pipe a ring of frosting flush with the edge of the cake. Fill the circle with a little more frosting, then spread out with an offset spatula to create an even layer.
Lay a second layer of cake on top, then repeat the process of piping a circle of buttercream flush with the edge of the second layer of cake. This time, fill the ring of buttercream with the jam and spread the jam evenly.
Add the final layer of cake, this time bottom side up so the top of the cake is completely flat and solid. Spread some of the remaining buttercream on top so you have an even layer, a few mm thick. Use the rest sparingly on the sides. You want it to be almost like a crumb coat, so some parts are barely covered.
Halve the strawberries (you can keep smaller ones whole) and use them to decorate the top of your cake along with the remaining raspberries. If you want to be especially decorative, you can add some flowers for wow factor. We opted for wild ones – just make sure they're edible!