8 Tbsp. (1 stick) unsalted butter, at room temperature
1 lb. dark brown sugar
4 oz. unsweetened chocolate, melted
1 tsp. vanilla extract
1¼ c. sifted cake flour
Pinch of salt
½ c. sour cream
2 tsp. baking soda
1 c. boiling water
Preheat the oven to 350 degrees. Line two tins that each have six star shapes with parchment paper. To cut the parchment paper to the correct size, turn over the pan to trace the outline of one star onto parchment paper. Once one is cut out, use that as a template to cut out the rest.
Lightly grease each pan with nonstick baking spray or melted butter using a pastry brush. Dust each pan with flour, knocking out any excess flour.
Place the parchment cutouts into the bottom of each pan, lightly spraying or greasing the parchment with melted butter, and then dusting with flour. (To make removing the cakes as easy as possible, use silicone pans if available.)
Using the paddle attachment in a large bowl of an electric mixer, cream together the butter and sugar at medium speed until the mixture is very light in color, about five minutes. With the mixer turned off, add the chocolate and resume mixing, using a spatula to scrape down the sides of the bowl. Add the eggs one at a time, followed by the vanilla, mixing well after each addition. Combine the flour with the salt in a medium bowl. In a small bowl, whisk together the sour cream, baking soda and boiling water until smooth.
Alternating between the sour cream mixture and the flour and salt mixture, add half of each to the chocolate and butter mixture and mix at medium speed until just combined. Pour the batter into the cake pans so they are no more than half filled. Bake for about 20 minutes, or until a toothpick inserted in the center of the cake comes out clean. Remove the pans to a cooling rack and frost the cakes once they have reached room temperature.