Preheat your oven to 275°F. Butter the bottom and sides of a 9-inch springform pan; set aside.
In a microwave-safe bowl, combine the butter and semisweet chocolate. Heat in 30-second intervals, stirring after each, until melted and smooth. Let cool slightly.
Whisk the egg yolks into the melted chocolate mixture until combined.
In a separate bowl, beat the egg whites until soft peaks form. Gradually add the sugar while beating, until glossy stiff peaks form.
Gently whisk 1/4 of the beaten egg whites into the chocolate mixture to lighten it. Then, using a spatula, fold in the remaining egg whites gently, taking care not to deflate the mixture.
Pour the batter into the prepared pan. Smooth the top with a spatula.
Bake for 45-50 minutes, or until the cake pulls away from the edges of the pan and the center is set. Let cool completely on a wire rack.
Meanwhile, prepare the whipped cream by beating the heavy cream, powdered sugar, and vanilla extract together until stiff peaks form. Chill until ready to use.
Remove the sides of the springform pan, slice the cake, and serve with a dollop of whipped cream. Enjoy!