Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream the softened butter and sugar together until light and fluffy.
Beat in the eggs one at a time, followed by the vanilla extract.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Mix until just combined.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan.
In a heatproof bowl, melt the chopped chocolate and butter together over a pot of simmering water, stirring until smooth. Remove from heat and let cool slightly.
In a separate bowl, beat the egg whites until soft peaks form. Gradually add the sugar and continue beating until stiff peaks form.
Stir the egg yolks into the cooled chocolate mixture until well combined.
Gently fold the whipped egg whites into the chocolate mixture until no white streaks remain.
In another bowl, whip the cream.
