Preheat oven to 180 C / Gas 4. Line the bottom of a greased 23cm springform tin with baking parchment, then grease the baking parchment.
In a food processor, puree the chestnuts with the butter and the rum, scraping down the sides, until the mixture is smooth. Add the melted plain chocolate and blend the mixture until it is combined well. With the motor running, add the yolks, 1 at a time and transfer the mixture to a large bowl.
In a bowl with an electric mixer beat the whites with the salt until they hold soft peaks. Add the sugar, a little at a time, beating and beat the meringue until it holds stiff peaks.
Whisk about a quarter of the meringue into the chocolate mixture to lighten it and fold in the remaining meringue gently but thoroughly. Pour the batter into the prepared tin and smooth the top.
Bake the cake in the middle of a 180 C / Gas 4 oven for 45 to 55 minutes or until a tester comes out with crumbs adhering to it and the top is cracked. Let the cake cool in the tin on a rack for 5 minutes, remove the side of the tin and invert the cake onto another rack. Remove the bottom of the tin, invert the cake onto a rack and let it cool completely. (The cake will sink as it cools.)
To Make Glaze: Put 170g of the finely chopped chocolate in a small bowl. In a saucepan, bring 125ml cream to the boil and pour it over the chocolate. Stir the mixture until the chocolate is melted and the glaze is smooth and stir in 1 tablespoon of the rum. Dip each marrons glace halfway into the glaze to coat it partially, transfer the chestnuts to a foil-covered tray and let them set.
Invert the cake onto a rack set on greaseproof paper, pour the glaze over it, smoothing the glaze with a spatula and letting the excess drip down the side and let the cake stand for 2 hours or until the glaze is set. Transfer the cake carefully to a serving plate and garnish it with the coated chestnuts.
Make the whipped cream just before serving the cake: In a chilled bowl with chilled beaters beat the 250ml double cream until it holds soft peaks, beat in the 2 tablespoons sugar and the1 tablespoon rum and beat the mixture until it holds stiff peaks. Fold in the chopped marrons glace. Serve the cake with the whipped cream.
Lovely texture with the chestnuts and looks very heavy. Flavour-wise it is very bitter, however that could be because I used 74% dark chocolate, so maybe a 50% would be a bit better or perhaps increase the sugar content.
You definitely need to have the cream with it to balance the bitterness. On the whole though a nice cake and I would make it again with some tweaks! - 03 Nov 2011