Preheat oven to 375°F.
Grease ramekins by taking 1 TB melted butter and brushing the whole interior and tops (you can also use a paper towel if you don't have a pastry brush). Then dust over the butter with sugar, about 1 TB of sugar per ramekin.
Chocolate Base: Melt the chocolate, and set aside.
Heat heavy cream on medium-low over the stove, then add cocoa and whisk together well. Set aside.
Separate egg yolks and whites. You will have one extra yolk - you can save in the fridge to use later or toss. Add about half of the sugar to the yolks and whisk together to thicken. Then add vanilla and salt, and mix again.
Add a drizzle of the hot cream to the egg yolks to temper, or "shock" the eggs. Then add the yolks to the remaining cream and cocoa in the saucepan. Whisk well the entire time to prevent eggs from scrambling.
Stir in the melted chocolate to the yolk mixture over low heat until thickened. Set aside to cool.
Meringue: While the chocolate cools, make the meringue. Start by using a mixer to beat 3 egg yolks on low until frothy. Then add cream of tartar. Turn speed up to medium and gently add in remaining sugar. Turn up speed to high and beat for about 5 minutes, until stiff peaks form.
Take about 1/4 of the meringue and add it to the cooled chocolate base. This will help lighten up the base to make it easier to fold into the rest of the meringue without deflating it.
Assembly: Once that portion is mixed together well, add remaining meringue and FOLD (not mix) together.
Put the mixture in the 4 small ramekins. Fill till it's almost to the top.
Bake for 18-20 minutes. Don't open the oven door until time is up. Doing so will cause your souffle to fall.
Serve immediately with powdered sugar, ice cream, or freshly whipped cream.