Preheat the oven to 375°F. Butter 4 6-ounce ramekins generously.

Place the butter and chocolate in a large microwave safe bowl and microwave in 30-second increments until completely melted, stirring after each increment.

Use an electric mixer to whisk the egg yolks, vanilla and salt into the melted butter and chocolate until well combined, set aside.

Wash the electric mixer beaters well. Then beat the egg whites and cream of tartar in another clean bowl until they hold soft peaks, about 2 minutes. Continue beating and gradually add the sugar until the egg whites are very stiff and glossy.

Gently fold the whipped egg whites into the chocolate mixture a little portion at a time until combined, being careful not to deflate the egg whites.

Transfer into the prepared ramekins. Bake for 13-15 minutes or until the center is nearly set. Dust with powdered sugar before serving.