Preheat oven to 375F. Prepare the ramekins by greasing the inside with softened butter then dusting with sugar. Tap out excess sugar and set prepared ramekins aside.
Using a double boiler, melt the chocolate and the butter over medium heat, stirring occasionally.
In a bowl, whisk together the egg yolks, vanilla, and salt.
Once the chocolate and butter mixture is fully melted, whisk into the egg yolk mixture. Set aside.
Using a handheld mixer or stand mixer fitted with the whisk attachment, whisk the egg whites and cream of tartar on high speed until soft peaks form. Once formed, continue whipping and gradually add the granulated sugar. Once all sugar is added, beat until stiff peaks. Do not over-beat.
Fold the egg whites into the chocolate mixture in 3 additions, making sure to fold carefully.
Refrigerate the batter for 10 minutes to allow to rest. Once rested, evenly spoon the batter into the 4 prepared ramekins. Carefully smooth the top and run a paring knife around the rim of the ramekin to lightly release the batter.
Bake for 13-14 minutes, or until the center just barely jiggles. Serve immediately.
