Preheat the oven to 375°F / 190°C and grease a 20 cm springform or cake pan generously with butter. Dust with flour and tap out the excess. (If using a cake pan without a removable bottom line the base with parchment paper).

In a large heatproof bowl, combine the butter and chocolate. Set over a saucepan of gently boiling water and heat, stirring often, until nearly completely melted. Remove bowl from saucepan, whisk mixture until smooth and set aside.

In a mixing bowl, whisk the egg yolks with the Mascobado sugar and salt until well combined. Stir in the warm chocolate and set aside.

Put the egg whites in a large mixing bowl with the lemon juice and beat until fluffy, then add the raw cane sugar and beat until the whites form peaks.

Stir the ground chestnuts into the chocolate batter, then fold in 1/4 of the egg white mixture. Gently fold in remaining whites and transfer batter into the prepared springform pan, smoothing the top with a spatula.

Bake for 30-35 minutes, until a toothpick inserted in the center of the cake comes out almost clean. Set aside and cool on a wire rack. The cake may sink and crack a bit on top — this can be expected. Run a knife around the edge of cake to free sides and remove form. Transfer to a serving plate, sprinkle with cocoa powder or icing sugar. Or garnish as desired and enjoy!

Store the cake covered, at room temperature, for up to 3 days.