Preheat the oven to 375 degrees F.
Butter the ramekins and pour 1 tablespoon of granulated sugar in each. Turn them to let the sugar stick to the butter, and then pour out the excess. Place the ramekins on a sheet pan.
Using a stand mixer, start whisking the egg whites on low speed. Slowly add 1 1/3 cups of the granulated sugar and the cream of tartar. Continue to whisk until medium peaks form.
Melt the chocolate with the 2 sticks butter in a double boiler, stirring continuously for about 7 minutes. Then, remove the chocolate from the double boiler and add the egg yolks, one at a time, stirring until smooth. Fold the egg white mixture into the chocolate a little at a time. Add the batter to the buttered and sugared ramekins, filling them to the top. Bake for about 15 minutes.