Butter and line the base and sides of a round, loosebottomed 23cm cake tin with non-stick baking paper. Make the Easy vanilla cake batter (see 'Goes well with'), spread half of it into the tin, then mix half the berries into what is left. Spoon this over the first layer and smooth the top. Bake for 1 hr 30 mins; check with a skewer - it should come out clean.
Cool for 30 mins, then remove from the tin and leave to cool on a wire rack. Once completely cold, put onto a plate, smooth the crème fraîche over and pile the remaining berries on the top. Dust with icing sugar, if using. The cake is best eaten fresh, but will keep in the fridge for up to 2 days; return to room temperature before topping with the cream and fruit and serving.