Canned chestnut purée comes in different sizes and weights depending on which country produces it. We tested the recipe with cans of slightly varying sizes and it did not make much difference to the cake. The most commonly available can size is the one in this recipe.6 to 8 servings
4 oz (125 g) bittersweet chocolate
3/4 cup (175 mL) granulated sugar
4 eggs, separated
One can (440 g) chestnut purée
6 oz (175 g) bittersweet chocolate
3 tbsp (45 mL) softened butter
1/3 cup (75 mL) Mascarpone
2 tbsp (25 mL) rum
Candied chestnuts, optional
1. Preheat oven to 350°F (180°C).
2. Line the base of a 9-inch (2.5-L) springform pan with parchment paper. Butter sides.
3. Melt chocolate on top of double boiler or heavy pot on low heat. Remove from heat to cool slightly.
4. Reserve 2 tbsp (25 mL) sugar. With an electric mixer, beat egg yolks and remaining sugar for about 3 minutes or until tripled in volume. Beat in cooled chocolate and chestnut purée.
5. Beat egg whites until frothy, then beat in reserved 2 tbsp (25 mL) sugar, whisking until mixture is thick and glossy, about 6 to 7 minutes. Stir one quarter of egg whites into cake, then gently fold in remaining egg whites. Spoon into springform and bake for 35 to 40 minutes or until a cake tester comes out clean. Cool completely in pan then turn out onto a wire rack.
6. To make frosting, melt chocolate in heavy pot over low heat. Beat in butter, mascarpone and rum. Remove from heat. Cool for 15 minutes, then frost the cake with it.
7. Refrigerate cake overnight before serving. Decorate with candied chestnuts, if desired.