Heat the oven to 180C/160C fan/gas 4. Butter a 20cm cake tin and line with baking parchment. Beat the butter and sugar together using an electric whisk until pale and fluffy, about 5 mins. Beat in the eggs, one at a time, then fold in the flour, baking powder and milk. (If the mixture starts to split when adding the eggs, add 1 tbsp of the flour with each one.) Scrape half the batter into a second bowl. Stir the raspberry powder and food colouring into one bowl and the rosewater into the other, so you have a bowl of pink berry batter and one of plain rose batter.

Alternately spoon the batters into the middle of the tin (about 1-2 tbsp each time) until all the batter is used up and it has filled the tin. If the batter is quite thick, gently press in each spoonful to encourage it to the edge. Use a skewer to swirl the batters together to create a marbled effect. Bake for 40-45 mins until golden, risen and a skewer inserted into the middle comes out clean. Cool on a wire rack.

Meanwhile, put the fresh or frozen berries and icing sugar in a bowl and leave for 1 hr until syrupy. Add a drop of rosewater, if you like. Serve with slices of the cake alongside the crème fraîche.