For the icing
200g icing sugar, sifted
2 tbsp milk
50g walnut broken or roughly chopped
1. Heat oven to 180C/160C fan/gas 4 and line a 20 x 30cm brownie tin with baking parchment. Tip the butter and chocolate into a non-stick saucepan and melt over a low heat, stirring every now and then, until smooth – take care not to overheat it. Leave to cool.
2. Meanwhile, mix the sugar, flour, cocoa and almonds with the xanthan gum. Stir into the chocolate mixture, then beat in the eggs and add the walnuts. Turn the mixture into the tin, level with a spatula and bake for 30-35 mins until firm and just cooked – when tested with a skewer, the crumb should still be a little moist. Cool in the tin.
3. To make the icing, melt the butter in a non-stick pan. Stir in the cocoa and cook, stirring, for about 1 min. Add the icing sugar and milk, and beat well. Pour on top of the cooled cake, scatter with the walnuts and lightly press them in. Leave until set, then carefully remove from the tin, strip off the baking parchment and cut into squares. Will keep for several days in a tin.