1. Preheat the oven to 190C/375F/gas mark 5. Butter and flour the sides of a 23cm/9in spring-release tin and place a piece of greaseproof paper in the base.
2. Place the butter and chocolate in a large bowl over a saucepan of simmering water until melted. Remove from the heat and beat in the sugar and egg yolks.
3. Whisk the egg whites until they reach soft peaks. Fold the ground almonds, lemon zest and egg whites into the chocolate mixture and transfer to the prepared tin.
4. Cook in the centre of the oven for 45-50 minutes until risen and firm to the touch. Cool in the tin, transfer to a wire rack, cover with a clean tea towel and leave until cold. Dust with cocoa powder or icing sugar and serve with mascarpone.
Quick & Easy: 30 minutes or less
Healthy Recipe: low fat and/or calories
Kid-Friendly: extra nutrition and/or fun!
Low-Carb: low carbohydrates
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