
Preheat oven to 350 degrees. Spray a 9-inch by 4-inch loaf pan with baking spray with flour.
Sift flour 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.
Cream butter or margarine with sugar in the mixer, 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. Turn out of the pan and let cool completely on rack.
Put the remaining 3 ounces chocolate in a medium bowl. Heat cream or topping 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.
