Preheat the oven to 325 degrees F. Brush a 10-inch Bundt pan with the room-temperature (but still firm) unsalted butter. Pour 1/2 cup sugar into the pan, swirling the sugar in the pan so that the chimney and sides are covered well. Turn the pan upside down to remove any excess sugar and set aside.

Sift together the flour, cocoa, baking powder, baking soda, and salt.

Into the bowl of an electric mixer fitted with a paddle, cream the butter on medium speed. Add the sugar and beat until the mixture is light and fluffy. Scrape down the bowl with a rubber spatula. Beating at medium speed, add the eggs, one by one, beating only until they are well blended.

Beating on low speed, add a third of the flour mixture and a third of the buttermilk. Do this twice more with the remaining dry ingredients and buttermilk, scraping the bowl with a rubber spatula in between. Finally, beat the batter on medium speed for 30 seconds, until it is smooth.

Pour the batter into the prepared pan. Bake the cake for 50 to 60 minutes, until the top feels springy and a toothpick inserted in the middle comes out clean.

Cool the cake in the pan on a wire rack for about 20 minutes. Then turn it out onto the wire rack to finish cooling.

To finish, dust the cake with cocoa or confectioners' sugar.