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First make the blackberry sauce to ripple through the mallow. Put the berries and 1 tsp sugar in a small bowl and cover with cling film. Microwave for 30 secs on High. Alternatively, add 1 tsp water then soften in a pan over a low heat. Mash well until saucy, then leave to cool.
Heat oven to 160C/140C fan/gas 3. Grease and line the base and sides of a 23cm square traybake tin, leaving some overhang. Using electric beaters or a hand whisk, beat the butter and sugar in a large bowl until very pale and creamy, then beat in the egg and yolks, followed by the vanilla. Mix the flour, almonds, baking powder, nutmeg and a pinch of salt, then fold into the fluffy mix to make a very thick batter.
Fold in the apple, then spoon into the tin and smooth over the top. Scatter with the blackberries, poke them in just a little, then bake for 45 mins until golden and a skewer inserted into the middle comes out clean. After removing, leave the oven on.
For the mallow topping, you’ll need a large piping bag with a 1cm nozzle, or a food bag with a corner snipped off. Whisk the egg whites, lemon juice and a pinch of salt to stiff peaks. Add the sugar 1 tbsp at a time, whisking well after each spoonful, to make a shiny, stiff meringue. Ripple with the cooled blackberry mix, then spoon into the bag. Pipe 16 evenly spaced, walnut-sized meringues on top of the cake (you’ll have some left over), scatter with the toasted almonds, then bake for 10-12 mins until the meringues are just set. Cool in the tin for 30 mins, then remove to a rack to cool completely. Add a dusting of icing sugar, if you like, then cut into squares.