For the cake
Preheat the oven to 275°. Butter the bottom and sides of a 9" springform pan. Set aside.
Place butter and chocolate in a large, microwave safe bowl and microwave in 30-second increments, stirring after each 30 seconds, until completely melted. You can also use a double boiler to melt the butter and chocolate. Allow the mixture to cool for about 10 minutes or until no longer very hot to the touch.
Add the egg yolks to the cooled butter and chocolate mixture, and whisk until smooth.
In a separate medium bowl, beat egg whites until soft peaks form (about 4 minutes). Gradually add the granulated sugar, and continue beating until glossy stiff peaks form (about 2 minutes more).
Add about 1/4 of the volume of the whipped egg whites to the chocolate mixture, and whisk gently just to combine.
Add the remaining egg whites and use a silicone spatula to gently fold them in to the mixture just until no white streaks remain. Take care to deflate the egg whites as little as possible.
Pour the batter into the prepared pan, and smooth the top with an offset spatula so the cake bakes evenly.
Place the cake in the center of the preheated oven and bake until the cake pulls away from the sides of the pan and is risen and no longer shiny in the center, 45 to 50 minutes. The "toothpick test" won't work here.
Place the cake, still in the pan, on a wire rack and allow it to cool completely.
Run a butter knife or offset spatula around the edge of the pan to ensure that the cake is not clinging to the sides. Carefully remove the sides of the pan and, if possible, slide the cake off the bottom round of the pan and place it on a serving platter.
For the whipped cream
To a large mixing bowl, add the whipping cream, powdered sugar, and pure vanilla extract.
With a stand or handheld mixer fitted with the whisk attachment, beat on medium-high speed until stiff, but not dry, peaks form (about 4 minutes). Set aside.
Slice cake and serve with the homemade whipped cream.
To make dairy-free
Use a block-style vegan butter like Melt or Miyoko's. Note that I have not tried this yet, but think it will work! Also be sure your chocolate is dairy-free if needed.
About the pan size
If your springform pan is narrower than the 9" one I used, your cake will be much thicker, lofty, and will take longer to bake.
Serving: 1slice | Calories: 389kcal | Carbohydrates: 36g | Protein: 6g | Fat: 25g | Saturated Fat: 14g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 7g | Trans Fat: 0.4g | Cholesterol: 154mg | Sodium: 118mg | Potassium: 224mg | Fiber: 2g | Sugar: 31g | Vitamin A: 519IU | Calcium: 48mg | Iron: 2mg
Nutrition information is automatically calculated, so should only be used as an approximation.