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Grease a 23cm-deep, loose-bottomed cake tin and cut out a circle of baking parchment to fit into the bottom. Cut a thick strip of parchment to fit around the side of the tin. Heat oven to 190C/fan 170C/gas 5. Put the sugar and butter into a large mixing bowl. Using an electric whisk (or wooden spoon if you’ve got more time and energy), beat them together until creamy and soft. Crack in the eggs, one by one, giving a good beat after each addition. Stir in the vanilla extract, then the flour and baking powder, then stir through the milk to loosen the mixture. Carefully stir in half the raspberries, making sure you don’t mix too vigorously or they will fall apart.
Cook the cake for 25-30 mins until just about cooked through. Then, carefully take it out of the oven. Scatter the marshmallows all over the cake, trying to leave a 2cm border round the edge of the tin as the marshmallows will expand when cooking, then tuck raspberries into any gaps. Return to the oven for another 10 mins until the cake has fully cooked through and the marshmallows are lightly browned and gooey. Leave for 5 mins, then ease out of the tin, cut up and eat.