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Heat oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with paper cases. Roughly crush half the blackberries with a fork so the juice is running but there are still some chunky bits. Gently melt the butter and chocolate in a saucepan. Mix the flour, sugar, cocoa powder and a pinch of salt in a large mixing bowl. Put the kettle on.
Scrape the butter-chocolate mixture and eggs into the dry ingredients with the crushed blackberries and 100ml boiling water. Mix together until smooth, then add a little food colouring to boost the red/purple hue. Divide between the muffin cases and bake for 15 mins until risen and a skewer poked into the centre comes out clean. Lift from the tin onto a wire rack and cool completely.
Whizz the remaining 150g blackberries to a purée. Beat the butter, icing sugar and vanilla together until really pale and fluffy – about 5 mins. Sieve the remaining blackberry purée and gently fold into the icing. Spoon into a piping bag fitted with your favourite nozzle and pipe a generous amount onto each cupcake – removing the paper case first if you want to show the colour of the sponge. Top each with a fresh blackberry and enjoy.