1. Preheat the oven to 350°F (177°C) and generously grease a 9×5 inch loaf pan.
2. Make the cake batter: Whisk the flour, baking powder, and salt together. Set aside.
3. Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter and sugar together on medium-high speed until smooth and creamy, about 2 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the oil and beat until combined. The mixture will still be quite creamy. Add the eggs and beat on high speed for 1 minute and then beat in the sour cream and vanilla extract. Scrape down the sides and up the bottom of the bowl as needed. The mixture will be lumpy– that’s ok. Pour the dry ingredients into the wet ingredients. Turn the mixer to low speed and as the mixer runs, slowly pour in 2/3 cup (160ml) milk. Beat on low speed just until all of the ingredients are 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.
4. You’ll have around 3-3.5 cups of batter. Transfer a little less than half (just eyeball it, doesn’t need to be perfect) to another bowl. Stir in the remaining 2 Tablespoons (30ml) milk, the cocoa powder, and espresso powder.
5. Layer and swirl the batters: 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.
6. Bake for about 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, loosely tent with aluminum foil as it bakes.
7. Cool cake in the pan set on a wire rack for 1 hour before removing from the pan. Note that the cake slightly sinks as it cools and that’s completely normal. Feel free to continue cooling the cake directly on a wire rack or you can top it and slice it while it’s still slightly warm after 1 hour cooling inside the pan.
8. As the cake cools, make the ganache topping: 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 20-30 minutes or until slightly thickened.
9. Drizzle or spread topping on cake. Topping sets into a fudge-like consistency after several hours.
10. Cover leftovers tightly and store cake (with or without topping) at room temperature for 3 days or in the refrigerator for up to 1 week.
Freezing Instructions: Topped or plain cake freezes well up to 3 months. Thaw overnight in the refrigerator, then bring to room temperature or serve cold.
Oil: For best taste and texture, use vegetable oil. In a pinch, you can replace with canola oil or olive oil. The cake can taste greasy with melted coconut oil, but if you try it, it’s imperative the other ingredients are room temperature so the coconut oil doesn’t solidify as you mix the batter together.
Whole Milk & Sour Cream: Feel free to replace the sour cream with low fat plain yogurt. For best results, use whole milk. Lower fat or nondairy milks work in a pinch. I don’t recommend replacing the sour cream and/or milk with buttermilk in this recipe.
Room Temperature Ingredients: All refrigerated items should be at room temperature so the batter mixes together easily and evenly. Read more about room temperature ingredients in baking.
Espresso Powder: Espresso powder will not make the cake taste like coffee. Instead, it deepens the chocolate flavor. It’s optional, but for best taste, I highly recommend it. You can use the same amount of instant coffee (the powder) instead of espresso powder if desired.
Ganache Topping: For the pictured ganache topping, I used 4 ounces of a milk chocolate baking bar from Trader Joes called “Milk Chocolate Pound Plus.” Milk chocolate is softer than semi-sweet and thus produces a thinner ganache. If you use milk chocolate, sometimes sold as “sweet chocolate” or “German’s chocolate” by the brand Bakers, reduce the heavy cream down to 1/4 cup (4 Tbsp; 60ml). If using semi-sweet chocolate, use 6 Tablespoons (90ml) heavy cream as listed above to produce a nice thick ganache.