Preheat the oven to 350°F (177°C) and generously grease a 9×5-inch loaf pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium speed until light and fluffy, about 5 minutes. Scrape down the sides and bottom of the bowl with a silicone spatula as needed. Add the eggs and vanilla and beat on high speed for 1 minute. The mixture may look a bit curdled at this point—that’s OK! Scrape down the sides and bottom of the bowl as needed. Pour the flour mixture into the wet ingredients. Turn the mixer to low speed and as the mixer runs, slowly pour in 3/4 cup + 2 Tbsp (210g/ml) of the buttermilk (reserve remaining 2 Tbsp (30g/ml)). Beat on low speed just until combined. Do not over-mix. You may need to whisk it all by hand to make sure there are no large lumps at the bottom of the bowl. The batter will be slightly thick.

You’ll have about 3–3.5 cups of batter. Transfer a little less than half (just eyeball it, doesn’t need to be perfect) of the batter to another bowl. Warm the reserved 2 Tablespoons of buttermilk in the microwave for 10–15 seconds. Stir the espresso powder into the warm buttermilk until dissolved, then pour it into the bowl that contains slightly less than half of the batter. Sprinkle the cocoa powder over the top, then stir, whisk, or beat on low speed until combined.

Spread a thin layer of the plain vanilla batter in the bottom of your prepared loaf pan. Now you will layer spoonfuls of each batter on top—cover bottom vanilla layer with a few spoonfuls of the chocolate batter, then a few spoonfuls of the vanilla batter, then more chocolate batter, then more vanilla batter, and so on until all of the cake batter is used. Gently shimmy the pan to level out the batters. Using a knife, make rounded horizontal zig-zags from one side of the pan to the other, and then make rounded vertical zig-zags from the top to the bottom.

Bake for 65–75 minutes, or until a toothpick inserted in the center comes out clean. That time is a guideline—all ovens differ, so keep a close eye on the cake after 55 minutes. If the cake is browning too quickly on top before the center is fully baked, loosely tent a sheet of aluminum foil over top of the pan. (I always tent with foil after 30 minutes.)

Cool the cake in the pan set on a cooling rack for 30 minutes, then remove the cake from the pan and place it directly on the rack.

Place the chocolate and cream in a medium heat-safe bowl set over a medium saucepan of simmering water. (Or use a double boiler if you have one.) Do not let the bottom of the bowl touch the water. Stir frequently until chocolate has melted and ganache is smooth. Remove from heat and set aside at room temperature for 20 minutes or until slightly thickened.

Drizzle or spread topping on cake. Cake is delicious sliced and served slightly warm. The topping sets into a fudge-like consistency after several hours.

Cover leftovers tightly and store cake (with or without topping) at room temperature for up to 3 days or in the refrigerator for up to 1 week.