Preheat the oven to 350°F / 175°C and grease a 9 × 5in / 23 × 13cm loaf pan. Whisk 2 cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt.
Beat ¾ cup of softened unsalted butter with 1¼ cups of granulated sugar for about 5 minutes. Beat in 3 eggs and 1 tablespoon of vanilla extract.
On low speed, add the flour mixture in additions, alternating with all but 2 tablespoons of 1 cup of buttermilk, and mix just until combined.
Warm the remaining buttermilk in the microwave for 10-15 seconds and stir in ½ teaspoon of espresso powder until dissolved.
Move a little less than half of the batter to another bowl. Stir in the espresso mixture, then sprinkle over ¼ cup of Dutch-process cocoa powder and stir it in.
Spread a thin layer of the vanilla batter in the pan, then add spoonfuls of both batters in layers. Drag a knife through in rounded zig-zags, first horizontally, then vertically.
Bake for 65-75 minutes, tenting with foil if the top browns too fast, until a toothpick in the center comes out clean. Cool in the pan for 30 minutes, then turn out.
For the ganache, melt 115g of chopped semi-sweet chocolate with 6 tablespoons of heavy cream over simmering water. Leave for about 20 minutes, until slightly thickened, then spread over the cake.
