Melt the 4 Tablespoons of butter and the chopped chocolate together in a double boiler or microwave. 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 egg whites and cream of tartar on high speed until soft peaks form, about 2 minutes. Gradually add sugar in 3 additions, beating until stiff glossy peaks form, about 2 more minutes.

Gently fold the egg whites into the chocolate mixture in 3 separate additions.

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

Brush four 6-ounce ramekins with softened butter and sprinkle with granulated sugar.

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

Place ramekins on a baking sheet and 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.