We wanted to retool classic pound cake to make it ultra-chocolatey without compromising its hallmark velvety-soft crumb. For deep chocolate flavor we used mostly Dutch-processed cocoa powder, which incorporated seamlessly into the batter; a couple ounces of milk chocolate added richness without interfering with the cake's texture.
We bloomed both the cocoa and the bar chocolate in hot water to maximize their impact. For an accurate measurement of boiling water, bring a full kettle of water to a boil and then measure out the desired amount. The test kitchen's preferred loaf pan measures 8 1/2-by-4 1/2 inches; if you use a 9-by 5-inch loaf pan, start checking for doneness 5 minutes early.
CHOCOLATE POUND CAKE
Start to finish: 1 hour, 45 minutes 1 cup (5 ounces) all-purpose flour 1 teaspoon salt 3/4 cup (2 1/4 ounces) Dutch-processed cocoa powder 2 ounces milk chocolate, chopped fine 1/3 cup boiling water 16 tablespoons unsalted butter, cut into 16 pieces and softened 1 cup (7 ounces) granulated sugar
1/4 cup packed (1 3/4 ounces) light brown sugar
2 teaspoons vanilla extract
5 large eggs, room temperature
Adjust oven rack to lower-middle position and heat oven to 325 F. Grease and flour 8 1/2-by-4 1/2-inch loaf pan. Whisk flour and salt together in bowl.
Place cocoa and chocolate in bowl. Pour boiling water over cocoa mixture and stir until chocolate is melted and no dry streaks of cocoa remain. Let mixture cool for 5 minutes.
Using stand mixer fitted with paddle, beat butter, granulated sugar, brown sugar, vanilla, and cocoa mixture on medium-high speed until fluffy, 2 to 3 minutes. Add eggs, one at a time, and beat until combined. Reduce speed to low and add flour mixture in 3 additions, scraping down bowl as needed, until just combined (batter may look slightly curdled). Give batter final stir by hand.
Transfer batter to prepared pan and gently tap pan on counter to release air bubbles. Bake until toothpick inserted in center comes out clean, 1 hour to 1 hour 10 minutes, rotating pan halfway through baking. Let cake cool in pan on wire rack for 10 minutes. Remove cake from pan and let cool completely on rack, about 2 hours. Serve. (Cake can be stored at room temperature for up to 3 days or frozen for up to 1 month; defrost cake at room temperature.)
Nutrition information per serving: 499 calories; 256 calories from fat; 29 g fat (17 g saturated; 1 g trans fats); 197 mg cholesterol; 343 mg sodium; 53 g carbohydrate; 2 g fiber; 36 g sugar; 8 g protein.