Cooked plain chestnuts, unlike the über-rich marron glacé, have a dusty flavour that is helped in something sweet such as these brownies by marinating them first in a simple mixture of brown sugar, vanilla and rum. Here, the chestnuts are chopped beforehand, which helps the syrup to soak in and, after they're baked in the brownie, helps them to taste almost candied in the crumb.
240g tin cooked chestnuts
200g brown sugar
25ml dark rum
2 tsp vanilla extract
2 medium eggs, separated
200g unsalted butter
200g dark chocolate, broken into small chunks
100g plain flour
Line a deep, 20cm square tin with nonstick baking paper. Chop the chestnuts, and in a bowl stir them with 100g sugar, rum and vanilla.
In a clean bowl, beat the egg whites until they form soft white peaks, slowly beat in 100g sugar until you have a soft meringue, then beat in the egg yolks.
Melt the butter and chocolate in a bowl over a pan of simmering water, then remove from the heat and beat into the chestnuts and flour. Stir this into the meringue, then spoon into the prepared tin. Bake at 170C (150C fan-assisted)/335F/gas mark 3 for 20-25 minutes, until barely set in the middle, then leave to cool completely in the tin before cutting.