Preheat the oven to 350°F (175°C).

Grease a 9″ glass pie dish with ½ tbsp.

of the butter, dust with 1 tbsp.

of the flour, tapping out the excess, and set aside.

Cut the remaining butter into small pieces.

Melt the 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 the bowl from heat and stir in the sugar, then the remaining 2 tbsp.

of flour, then the egg yolks, stirring until well combined.

Set aside to cool briefly.

Meanwhile, beat the 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 the chocolate mixture.

Pour the batter into the prepared pie pan.

Bake the cake until a toothpick inserted in the center comes out with some moist crumbs still attached (the cake will rise and the top will crack as it bakes, then collapse on itself as it cools), about 30 minutes.

Transfer the cake to a wire rack to cool briefly.

Serve warm or at room temperature, with ice cream if desired.