In a saucepan over medium heat, melt the butter and chocolate together until the chocolate mixture is smooth. Set aside.
Meanwhile, beat together the eggs and sugar with a whisk or electric beater until pale and thick, about 5 minutes.
Pour the melted chocolate in the egg mixture and mix well. Blend in the flour, just until combined.
Butter and lightly sugar 4 (4-ounce) molds, custard cups or ramekins. Tap out the excess sugar. Divide the batter among the molds. At this point you can refrigerate the desserts until you are ready to eat, for up to several hours, then bring them back to room temperature before baking.
Preheat the oven to 400 degrees F.
Bake the molten chocolate on a sheet tray for 8 to 10 minutes, the center will still be quite soft, but the sides will be set. Let rest for 5 minutes before serving with your favorite ice cream.
Instead of coating the molds with sugar, you can use crushed pistachios or other nuts.