Prepare the Oven and Cake Pan:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and grease an 8-inch round cake pan with butter. Line the bottom with parchment paper for easy removal later.
2. Prepare a Double Boiler: Set up a double boiler by placing a heatproof bowl over a pot of simmering water. Make sure the bowl doesn’t touch the water.

Melt the Chocolate:

3. Melt the Chocolate and Butter: In the prepared double boiler, melt the dark chocolate and butter together, stirring occasionally until smooth. Once melted, remove the bowl from the heat and stir in the vanilla extract.
4. Cool Slightly: Let the chocolate mixture cool for a few minutes while you work on the meringue.

Make the Meringue:

5. Beat the Egg Whites: In a separate, clean bowl, use an electric mixer to beat the egg whites and cream of tartar on medium speed until soft peaks form.
6. Add the Sugar: Gradually add the granulated sugar, a tablespoon at a time, and continue beating on high speed until stiff peaks form and the meringue is glossy.
7. Fold in Powdered Sugar: Gently fold in the powdered sugar until well combined.

Combine the Mixtures:

8. Mix the Egg Yolks into Chocolate: Stir the egg yolks into the slightly cooled chocolate mixture until fully incorporated.
9. Fold in the Meringue: Gently fold the meringue into the chocolate mixture. Be careful not to deflate the meringue. This will create the airy, fluffy texture.

Bake the Cake:

10. Pour the Batter into the Pan: Pour the batter into the prepared cake pan and smooth the top with a spatula.
11. Bake the Cake: Bake for 60-75 minutes, or until the cake is set in the center and a toothpick comes out with a few moist crumbs. The top will crack slightly, creating the signature meringue texture.
12. Cool in the Pan: Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.