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Heat oven to 190C/fan 170C/gas 5 and butter two 20cm sandwich tins. Line the bottom of the tins with baking paper. In a large bowl, beat the butter and 225g caster sugar together until creamy and light. Gradually add the egg, little by little, until all the egg is worked in and the mix is pale and fluffy. If the mix looks like it’s starting to split, add 1 tsp of the flour, then carry on.
Put the polenta in another bowl, then stir in the flour and baking powder. Beat the vanilla extract and zest into the eggy mix, then fold in the dry ingredients. Spoon half the batter into each tin and level the top. Scatter all but a handful of the raspberries over the mix and poke in gently. Sprinkle one of the sponges with the 1 tbsp sugar. Bake for 20 mins until risen and golden, but still with a little wobble under the crust.
Open the oven, whip out the sugar-crusted sponge and quickly poke the remaining frozen raspberries into the top. Bake both sponges for 10 more mins or until springy in the middle. If this sounds too tricky, just leave the sponges to bake for 30 mins – the cake won’t look as glam, but will still taste great (you can add the leftover berries to the filling instead). Cool in the tin for 10 mins, then cool completely on a rack. Be careful when turning out the raspberry-topped sponge and slide it off its base rather than turning it upside down.
When the sponges are cold, beat the soft cheese with the icing sugar, lemon zest and a little of the juice to loosen if it needs it. Very lightly whip the cream so that it just holds its shape, then fold into the cheese. Fold in the defrosted raspberries. Use to sandwich the sponges together, sugar-crusted on top, and serve dusted with more icing sugar.