1

Preheat the oven to 350°F. Butter and flour two 9-inch round cake pans. Place the cream in a small saucepan and bring it to a simmer over medium heat. Pour it over the chocolate in a bowl large enough to hold all the finished cake batter. Let sit for about 5 minutes, or until all the chocolate has melted, then whisk the mixture until there are no lumps. Stir in the sour cream and vanilla.

2

In a bowl, whisk together the flour, baking powder, baking soda, and salt.

3

In a stand mixer fitted with the paddle attachment, beat 3/4 cup of the sugar with the butter for about 2 minutes. Switch to the whisk attachment and beat for 5 minutes more, or until soft and creamy.

4

Beat in the egg yolks, one by one. Beat in the chocolate mixture until well combined. Transfer the chocolate-egg yolk mixture back to the large bowl.

5

Sift the dry ingredients over the mixture and fold with a rubber spatula just until there are no lumps.

6

In a stand mixer fitted with the whisk attachment, beat the egg whites and the cream of tartar (if using) at medium-high speed for about 2 minutes, until medium peaks form. Add the remaining sugar, then beat for about 3 minutes more, or until stiff peaks form. Whisk one-fourth of the egg white mixture into the chocolate mixture to lighten it.

7

Fold in the rest of the white mixture, just long enough so you no longer see streaks of egg white. Transfer the batter to the prepared cake pans and smooth the tops with an offset spatula.

8

Bake the cakes for about 35 minutes, or until they bounce back to the touch or a toothpick inserted in the center comes out clean. Let cool for 5 minutes in the cake pans and then turn out onto cake racks.