Preheat the oven to 350°F / 175°C. Generously butter a 7" springform pan and coat it with sugar.
Add the cocoa powder and instant espresso (if using) to a medium sized mixing bowl.
Heat the milk in a large microwave safe bowl or liquid measuring cup until it's hot and starts to foam up (you'll want it to be larger bowl/cup to avoid the milk spilling over into your microwave)
Immediately pour the hot milk over the cocoa powder. Stir the mixture together until it's cohesive and there's no lumps.
Add in the vegetable oil, sugar, egg, and sour cream. Whisk vigorously until combined.
Add in the flour, baking powder, and salt. Gently fold the dry ingredients into the wet ingredients until only a few flour streaks remain, then fold in the cherries just until no streaks of flour remain.
Pour the chocolate cake batter into your prepared 7" cake pan. Bake for 30-35 minutes. To check for doneness: I recommend using a thermometer for the best results: when stuck into the middle of the cake the thermometer should read 205°F / 96°C. The cake is also done when the center no longer jiggles and the top becomes more firm.
Allow the cake to rest in the cake pan for at least 15 minutes before transferring it out to a cooling rack.