6 ounces bittersweet chocolate, cut into small chunks
2 1/4 cups buttermilk
1 1/2 cups sour cream
1 teaspoon vanilla extract
1 teaspoon chocolate extract, optional
Preheat oven to 350 degree F and place a rack in the center of oven. Coat inside of the pan with melted butter using a pastry brush or paper towel. Sprinkle the sides and center of pan with turbinado sugar.
Toast walnuts in the oven for 7 to 8 minutes, stirring often until the nuts are light golden. Remove from oven and cool on rack.
Sift the flour, cocoa powder, and baking soda over a bowl. Add brown sugar, salt, and nutmeg, and toss to blend completely. Add the walnuts and bittersweet chocolate chunks and stir to blend thoroughly.
In a bowl of electric stand mixer, blend thoroughly buttermilk, sour cream, vanilla, and chocolate extract (if using) with a paddle attachment. Add the flour mixture in 4 stages, blending well after each addition. Stop often to scrape down the sides and bottom of the bowl with a rubber spatula.
Transfer the batter to the prepared pan, then use a rubber spatula to smooth and even the top. Bake for 35 to 38 minutes, until a cake tester inserted in the center of the cake comes out clean.
Remove the pan from the oven and cool on a rack for 20 minutes. Invert the pan onto the rack and lift the pan off the cake. Let the cake cool completely on the rack.