Preheat the oven to 350°F / 175°C. Butter and line an 9x5" metal** loaf pan with parchment paper.
In a medium-sized bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or with a hand mixer, beat together the butter, oil, and sugar for 3-5 minutes until light and fluffy.
Scrape down the sides of the bowl, and add in one egg at a time, beating until combined after the two additions.
Add in the egg white, vanilla, and sour cream and beat until combined.
Alternate between adding the flour mixture and the milk. Fold in ½ of the flour mixture, then all of the milk, followed by the rest of the flour mixture.
Add the cocoa powder to a small mixing bowl, then brew the espresso and immediately pour it into the cocoa powder, mixing until the mixture is smooth. Add 2 cups of the vanilla cake batter to the cocoa mixture, and gently mix the batter together just until no streaks remain.
To the vanilla cake batter, gently fold in the cherries.
Alternate scooping in the different batters into the loaf pan. Once all the batter has been used, use a knife to gently swirl the batters together.
Bake for 50-60 minutse, or until a knife inserted in the middle of the cake comes out with just a few moist crumbs.
While the cake is cooling, make the ganache.