Preheat the oven to 350 degrees and butter 4 4-inch ramekins, then place them on a sheet pan.
Gently melt the butter and chocolate together in a bowl over a double boiler, stirring occasionally. You can also melt them in the microwave at half power in 15 second intervals, stirring after each. Let cool slightly.
Whisk together the egg yolks and 3 tablespoons of sugar until light and frothy.
Add the cooled melted butter and chocolate mixture, then whisk to combine.
Use an electric mixer or stand mixer to whisk the egg whites and the remaining 3 tablespoons sugar until they form stiff peaks (when they stand straight up and don't collapse), about 3-5 minutes.
Use a rubber spatula to gently fold the egg whites into the chocolate mixture, being careful not to deflate.
Divide the batter evenly among the ramekins.
Bake for about 25-30 minutes, or until puffed up and just set in the middle (no longer jiggly).
Serve warm with a scoop of vanilla ice cream, or let them chill in the refrigerator and serve them cold with a dollop of whipped cream.