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

In a clean mixing bowl, beat egg whites and cream of tartar on high speed until soft peaks form. Add sugar gradually, continuing to beat until stiff, glossy peaks form.

Gently fold the egg whites into the chocolate mixture in three parts. Ensure each addition is fully incorporated before adding the next.

Refrigerate batter for 5-10 minutes while preparing the ramekins and preheating the oven.

Preheat the oven to 400°F (204°C). Brush four 6-ounce ramekins with softened butter and coat them with granulated sugar. Place ramekins on a baking sheet.

Spoon the batter evenly into the prepared ramekins. Smooth the surface and run a knife or spatula around the edges to create a channel.

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

Remove from the oven and serve immediately. Optional toppings include whipped cream, fresh raspberries, or a dusting of powdered sugar.