Heat oven to 400 degrees (if you are cooking right away). Generously grease the inside of four ½ cup ramekins with softened butter and dust with cocoa powder, tapping out any excess.
Melt the chocolate and butter together how you like - using a double boiler, or in the microwave. I prefer the microwave, using a 4 cup glass measure and heating for 45 seconds, then whisking until smooth. If using a double boiler, transfer the chocolate mixture to a medium sized bowl.
Stir in the sugar and flour. Then add the eggs and whisk until everything is incorporated.
Divide the batter evenly between the cups (they will be about 3/4 full) and set the cups on a small cookie sheet to make it easy to go in and out of the oven. See note for make ahead option.**
Bake for 15 minutes until tops are firm and edges are set. Invert immediately (or at least within five minutes) onto individual serving plates. Dust with additional cocoa powder, if desired.
Notes
*I use whole wheat pastry flour. A 1:1 gluten free flour will also work.**To Make Ahead: At this point, the cakes can be set aside at room temperature for an hour or so. Then simply bake them while you're eating so that they will be ready (and gooiest!) when you want to serve dessert. They can even be make the day before and kept, covered with plastic, in the refrigerator. When you're ready to serve them, let them sit on the counter at room temperature while the oven heats and then bake as directed.