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Heat the oven to 180C/160C fan/gas 4. Butter a 20cm square tin and line with baking parchment. Stir the melted butter and vanilla together in a bowl. Mix the caster sugar, flour and cocoa together in another bowl.
Pour the bowl of wet ingredients into the dry and beat together, then add the egg and beat again until combined. Press into the prepared tin. Brush the top with water and sprinkle over the granulated sugar.
Bake for 30-35 mins. Leave to cool slightly in the tin for 5 mins before slicing into squares and transferring to a wire rack to cool completely.