Melt the 4 Tablespoons of butter and the chopped chocolate together in a double boiler or microwave. Microwave in 20-second increments, stirring until completely melted, then cool for 3-5 minutes.

Whisk egg yolks, vanilla extract, and salt into the chocolate mixture. Set aside.

In a clean medium glass or metal mixing bowl, beat the egg whites and cream of tartar together on high speed until soft peaks form, about 2 minutes. Gradually add sugar in 3 additions, then beat until stiff glossy peaks form, about 2 more minutes.

Gently fold the egg whites into the chocolate mixture in 3 separate additions, combining each addition completely before folding in more.

Refrigerate the batter for 5-10 minutes while preheating the oven to 400°F (204°C).

Prepare four 6-ounce ramekins by brushing them with softened butter and sprinkling with granulated sugar.

Spoon batter evenly into ramekins, smoothing down the surface and creating a channel between the batter and the rim.

Place the baking sheet with ramekins in the oven and immediately reduce temperature to 375°F (191°C). Bake for 13-14 minutes until the edges are set and the center jiggles slightly.

Remove from the oven and serve immediately.