Heat the oven to 180C/160C fan/gas 4. Butter and line the base and sides of two 20cm sandwich tins.
Beat the butter and sugar together in a large bowl with an electric whisk for 8-10 mins, or until pale and fluffy. Add the eggs one at a time, beating well after each addition. Sift in the flour, baking powder and salt, and fold in using a large metal spoon. Add just enough of the milk to create a dropping consistency.
Divide the batter between the prepared tins and smooth the tops with a spatula. Bake on the middle shelf of the oven for 25-30 mins, or until golden and firm to the touch. Leave to cool slightly in the tins, then transfer to a wire rack to cool completely.
Meanwhile, make the macerated berries. Mix the sugar, lemon juice, vanilla and mint together until the sugar has dissolved. Gently stir in the strawberries and raspberries until coated in the mixture. Transfer to the fridge and chill for 30-45 mins until softened.
To make the crème diplomat, put the custard, vanilla pod and sugar in a pan set over a medium heat and bring to the boil, stirring occasionally. Mix the custard powder with 3 tbsp water to dilute and stir until smooth. Whisk into the boiling custard, then reduce the heat to a simmer and continue whisking for 2-3 mins until thick. Put in a heatproof bowl and leave to cool slightly, then chill until cooled completely. Whisk the double cream to soft peaks, and when the custard is cool, remove the vanilla pod and whisk in the cream until the mix is thick enough to pipe.
Drain the macerated berries, reserving the liquid. To assemble, spread half the crème diplomat over one of the sponges, then top with three-quarters of the berries. Invert the second sponge on top, so the flat base of the sponge is facing up, and dust with some icing sugar. Put the remaining crème diplomat in a piping bag fitted with a star nozzle and pipe rosettes around the edge of the cake. Dot the rest of the berries between the rosettes of cream, and serve with the reserved macerating liquid alongside – don’t pour it over before serving as it will split the crème diplomat. Will keep in the fridge for up to three days.