Preheat the oven to 375 F. Prepare the baking dishes (either 4 ramekins or 1 large soufflé dish) by lightly greasing with butter then pouring in sugar and tapping out the excess.
Begin whisking the egg whites on low speed, then slowly add the sugar and cream of tartar and continue to whisk until medium peaks form.
Melt the chocolate with the butter, either carefully in the microwave or over a double boiler. Remove from heat and let cook 30 seconds to 1 minute, then add egg yolks, one at a time. Stir until evenly incorporated.
Fold the egg white mixture into the chocolate mixture, working with about 1/3 the egg whites at a time.
Pour batter into ramekins, distributing equally. Bake until the top has puffed but the middle is still jiggly in the center. It'll take about 15 minutes for small ramekins, and about 25 minutes for a single dish.
Serve warm, dusted with powdered sugar or topped with whipped cream. They go great with fresh fruit as well.
