Use 1 tablespoon of the butter to grease the 4 ramekins. Coat the inside of the ramekins with the cocoa powder, tapping out the excess and re-using to coat the remaining ramekins.

In a large bowl over a pan of simmering water, melt the chocolate and remaining butter. Stir occasionally until smooth. Remove from the heat and add the vanilla extract. Whisk in the egg yolks one at a time until smooth. Set aside.

In another large bowl beat the egg whites with the sugar, salt, and cream of tartar until stiff peaks form. Using a metal spoon, gently fold the egg-white mixture into the chocolate. Divide the mixture between the 4 ramekins.

Bake until puffed and set, this will take about 10-15 minutes. Serve immediately.