10 oz bittersweet chocolate, chopped
1/2 pound (2 sticks) unsalted butter, softened 1 1/4 c granulated sugar
6 large eggs, separated
1/4 c orange liqueur, such as Grand Marnier
1 tb freshly grated orange zest (optional)
1 tsp pure vanilla extract
1 c all-purpose flour
1/4 tsp salt
Confectioner’s sugar, for dusting
Preheat oven to 350 F. Butter a 9” spring form pan or spray it with nonstick spray. Line the bottom of the pan with parchment paper.
Melt the chocolate in a heatproof bowl set over a pan of simmering water. Set aside to cool slightly.
In a large bowl, beat the butter and 1 cup of sugar with an electric mixer until light and fluffy, about two minutes. Beat in the egg yolks, orange liqueur, orange zest and vanilla extract. Stir in the melted chocolate. Stir in the flour and salt until just combined.
In another large bowl, beat the egg whites and the remaining 1/4 cup of sugar until soft peaks form, about four minutes. Stir one-third of the egg whites into the batter to lighten it. Carefully fold in the reaming egg whites in two additions.
Pour the batter into the prepared pan and bake for 35 to 40 minutes, until the top of the cake is dry and a cracked toothpick inserted in it comes out clean, but a toothpick in the center comes out lightly coated with chocolate. Cool the cake slightly; unmold and dust with confectioners’ sugar before serving.