Chocolate Marble Bread With Chocolate Ganache

8 ounces (2 sticks) unsalted butter, softened, plus more for the baking pan
1 1/4 cups all-purpose flour, plus more for dusting
1/2 teaspoon salt
3/4 teaspoon baking powder
8 ounces semisweet chocolate, coarsely chopped
2/3 cup plus 2 T sugar
4 large eggs
1/2 cup whole milk
1/4 cup heavy cream

Preheat oven to 350 degrees. Butter an 8 1/2-by-4-by-2 1/2-inch loaf pan; dust with flour, tapping out excess.

Sift flour, salt, and baking powder into a medium bowl.

Melt 5 ounces chocolate in a heatproof bowl set over a pan of simmering water, stirring occasionally until smooth.

Put butter in the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until smooth, 2 to 3 minutes. Add sugar; raise speed to medium. Cream butter and sugar until pale and fluffy, 3 to 4 minutes. Scrape down sides of bowl. Mix in eggs, 1 at a time. Reduce speed to low. Add flour mixture in three batches, alternating with two batches of milk. Mix until just combined.

Pour half of batter into melted chocolate; stir well. Alternating between remaining plain batter and the chocolate batter, drop large spoonfuls of batter into the prepared pan. When pan is filled, use a table knife to cut through mixture with a swirling motion.

Bake until a cake tester inserted into center comes out clean, about 1 hour. (If bread browns too quickly, tent with foil.) Let bread cool slightly in pan on a wire rack, about 10 minutes. Run a thin knife around edges of bread to loosen; unmold. Let cool completely on rack.

Make ganache:
Put the remaining 3 ounces chocolate in a medium bowl. Heat cream in a small saucepan over medium heat until just about to simmer. Pour cream over chocolate; stir until mixture is smooth. Let stand 10 minutes to thicken slightly.

Using a small offset spatula, spread ganache over cooled cake; let stand until set, about 1 hour.

Bread can be stored in an airtight container at room temperature, up to 2 days.