Preheat the oven to 180C/160C Fan/Gas 4. Grease 2 x 20cm/8in springform cake tins with butter and dust with flour.
In a bowl, beat the butter and 200g/7oz of the sugar together using an electric whisk until pale and fluffy.
Beat in the eggs one at a time, making sure each is fully incorporated into the mixture before adding the next. Add a little flour to the mixture if it starts to curdle.
Beat in the vanilla bean paste, then fold in the flour until just combined.
Divide the mixture equally between the prepared cake tins and smooth the tops.
Bake the cakes in the oven for 20-25 minutes, or until golden-brown and risen. A skewer inserted into the centre of the cakes should come out clean; if it doesn’t, return them to the oven for a further 5 minutes and check again.
Turn the cakes out onto a tea towel-lined wire rack and set aside to cool.
Meanwhile, dissolve the remaining 150g/5½oz caster sugar in 150ml/5fl oz water in a saucepan. Bring the mixture to the boil. Reduce the heat until the mixture is simmering, then simmer for 5 minutes, or until it resembles a light syrup.
Mix the berries and redcurrants together in a bowl, then stir all but a handful into the syrup, until they have softened but not broken down and the mixture resembles a compôte. Remove from the heat and set aside to cool.
When the cake and compote have both cooled, place one cake on a serving plate. Spread compôte over it, followed by the whipped cream, then spoon a little more compôte on top.
Gently set the second cake on top of the filling and dust the surface with icing sugar. Garnish the cake with the remaining berries. Serve immediately.