Molten Chocolate Cake

Preheat the oven to 375 °F. Liberally grease the ramekins with softened butter and dredge the inside with sugar. Line the prepared ramekins on a baking sheet and set aside. In the bowl of a stand mixer fitted with the whisk attachment, beat the eggs, egg yolks, and 3/4 cup sugar on medium-high speed until very pale and nearly tripled in volume (the foam will hold a "ribbon" when drizzled over itself), about 10 minutes. While the eggs are whipping, in a bowl set over a pan of barely simmering water, melt the chopped bittersweet and unsweetened chocolates with 1 1/4 cup butter. Keep warm. Transfer the egg foam to a large mixing bowl and add the melted chocolate. Gently fold until almost completely combined. Sprinkle the flour over the mixture and continue folding just until the flour is incorporated and the batter is blended. Divide the batter among the prepared ramekins, filling them almost to the top. Bake until the edges are set and the middle of the cake is starting to dry and sink slightly, about 10 minutes (take care not to overbake the cakes or the center will not be molten). Using a small sharp knife, loosen the edges of the cakes then invert each one onto a serving plate. Dust with confectioners' sugar, if desired, and serve warm.