An effortless flourless chocolate cake with intense dark chocolate flavor and a light and nutty texture. The cake is very moist and fudgy but not dense.
Serve with whipped cream
Preheat the oven to 350 °F (177°C /Gas Mark 4) oven. Butter an 8-inch (20 cm) springform pan. Set aside.
In a medium metal mixing bowl, add the broken-up chocolate, the butter and sugar to the bowl.
Add some water to a large 10-inch (25.5 cm) skillet just enough to cover the bottom of the pan. Set on a burner over medium heat. Bring the water to a gentle simmer and place the bowl with the chocolate, butter and sugar in the center of the skillet. Melt the chocolate and butter, stirring occasionally. Do not let the water get to a brisk boil. Keep it at a gentle simmer, being careful not to splash water into the chocolate.
Remove the bowl from the skillet just before all the butter has melted and stir until all the chocolate and butter has melted. Let the chocolate cool for four minutes.
Add the egg yolks one at a time to the chocolate, stirring between each addition until each yolk is incorporated.
In a separate bowl, with a stand mixer or hand-held mixer, beat the egg whites until stiff with soft peaks and still wet. Soft peaks will form when you lift out the beaters. Gently fold in the Grand Marnier if using.
Add the orange zest to the ground walnuts and mix together. Add the walnut mixture to the chocolate and half of the whipped egg whites. Fold into the chocolate. Then fold in the remaining egg whites.
Pour the batter into the prepared springform pan and place on a rimmed sheet pan in case it leaks. Place the cake in the oven and bake for 35 minutes.
Remove the cake from the oven and cool on a cake rack. Don't get upset if you see the cake deflate and crack as it cools. When the cake is completely cool, unlatch the pan and carefully remove the sides. Run an icing spatula, or thin sharp knife under the cake to loosen. It helps to clean off the spatula or knife every time you pull it out from under the cake. Carefully transfer the cake onto a serving plate.
Dust the cake with confectioners' sugar right before serving.
Right before serving, use a hand-held mixer and whip the heavy cream until it just holds its shape. Sprinkle in the confectioners' sugar and Grand Marnier, if using. Whip until combined soft peaks form. Taste and correct the whipped cream for sweetness and the Grand Marnier. Place in a small serving bowl.
Serve the whipped cream with the cake and extra fruit, like berries.
Before you start the cake, toast the walnuts in a preheated 350°F (177 °C / Gas Mark 4) oven. Spread a couple of handfuls (60 g) of the walnuts over a small rimmed baking sheet. Toast in the oven for 7 minutes. Spread the toasted walnuts over a clean lint free kitchen towel. Fold a portion of the towel over the nuts to cover and rub the towel with the walnuts back and forth to remove the walnut skin. No need to go crazy rubbing off all the skin. Rub back and forth a few times until no more skin comes off without scrubbing. Collect the walnuts leaving the loose skin behind and grind the walnuts in a food processor.