Preheat oven to 350°. Grease a 9" glass pie dish with 1⁄2 tbsp. of the butter, dust with 1 tbsp. of the flour, tapping out excess, and set aside. Cut remaining butter into small pieces. Melt butter and chocolate together in a medium bowl set over a pot of simmering water over medium-low heat, stirring occasionally with a wooden spoon. Remove bowl from heat and stir in sugar, then the remaining flour, then the egg yolks, stirring until well combined. Set aside to let cool briefly.
Meanwhile, beat egg whites in a medium mixing bowl with an electric mixer on medium-high speed until soft peaks form, 2-3 minutes. Using a rubber spatula, fold one-third of the egg whites at a time into chocolate mixture. Pour batter into prepared pie pan.
Bake cake until a toothpick inserted in center comes out with some moist crumbs still attached (cake will rise and top will crack as it bakes, then collapse on itself as it cools), 30 minutes. Transfer cake to a wire rack to let cool briefly. Serve warm or at room temperature, with ice cream, if you like.