For the cake:
2 1/4 cups all-purpose flour
1 cup unsweetened cocoa powder
2 tsp. baking powder
3/4 tsp. baking soda
1/4 tsp. salt
16 Tbs. (2 sticks) unsalted butter
1 1/2 cups granulated sugar
4 eggs, lightly beaten
1 tsp. vanilla extract
1 1/2 cups sour cream
6 oz. bittersweet chocolate, melted and cooled slightly
For the glaze:
1 cup granulated sugar
1/2 cup water
1 Tbs. chocolate liqueur
Have all the ingredients at room temperature.
Position a rack in the center of an oven and preheat to 325°F. Grease and flour a decorative 10-cup Bundt® pan.
To make the cake, over a sheet of waxed paper, sift together the flour, cocoa powder, baking powder, baking soda and salt; repeat until well blended. Set aside.
In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until smooth and creamy, 30 to 45 seconds. Reduce the speed to low and gradually add the granulated sugar, beating until blended. Increase the speed to medium-high and continue beating, occasionally scraping down the sides of the bowl, until the mixture is light and fluffy, 3 to 5 minutes. Add the eggs a little at a time, beating each addition until incorporated before adding more, until the mixture is thick and creamy, 1 to 2 minutes; stop mixer occasionally and scrape down the sides of the bowl. Beat in the vanilla.
Reduce the speed to low and fold in the flour mixture in three additions, alternating with the sour cream and beginning and ending with the flour, until just blended and no lumps of flour remain. Then gently fold in the chocolate.
Pour the batter into the prepared pan, spreading the batter so the sides are about 1 inch higher than the center. Bake until a toothpick inserted into the center of the cake comes out clean, about 1 hour. Transfer to a wire rack and cool the cake upright in the pan for 10 minutes.
Meanwhile, make the glaze: In a small saucepan over low heat, combine the granulated sugar and water and stir until the sugar is dissolved. Remove from the heat and stir in the chocolate liqueur.
Set the rack over a sheet of waxed paper, invert the pan onto the rack and lift off the pan. Generously brush the warm cake with the glaze. Let the cake cool to room temperature, then dust with confectioners' sugar just before serving. Top slices with a dollop of whipped cream.