Melt the 4 Tablespoons of butter and the chopped chocolate together in a double boiler or in a large heatproof bowl in the microwave, stirring every 20 seconds until completely melted and combined. Slightly cool for 3-5 minutes.

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

In a clean medium mixing bowl, beat the egg whites and cream of tartar on high speed until soft peaks form, about 2 minutes. Add the sugar gradually in 3 additions, beating for 5 seconds before adding the next Tablespoon. Once all sugar is added, continue beating until stiff glossy peaks form.

Gently fold the whipped egg whites into the chocolate mixture in 3 separate additions, ensuring each is fully incorporated before adding more.

Refrigerate the batter for 5-10 minutes as you preheat the oven to 400°F (204°C) and prepare the ramekins.

Brush four 6-ounce ramekins with softened butter and sprinkle granulated sugar inside, rotating each to ensure even coating. Place on a baking sheet and discard excess sugar.

Spoon the batter evenly into ramekins, smoothing the surface, and run a knife around the rim to create a channel.

Bake for 13-14 minutes at 375°F (191°C) until the edges are set and the centers jiggle slightly.

Remove from the oven and serve immediately.