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.