Preheat the oven to 350°F / 175°C. Line a muffin tin with 6 parchment liners.
Add the cocoa powder and the teaspoon of 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 a larger bowl/cup to avoid the milk spilling over into your microwave)
Immediately pour the hot milk over the cocoa powder. Give that all a good mix until the mixture is cohesive and has no lumps.
Add in the vegetable oil, sugar, egg, and sour cream. Whisk the mixture together until it's well combined.
Add in the flour, baking powder, and salt. Gently fold the dry ingredients into the wet ingredients just until the last streak of flour disappears into the batter.
Distribute the cupcake batter evenly between the six liners. Bake for 15-20 minutes. To check for doneness, I recommend using a thermometer for the best results. Stick a thermometer into the center of the cupcakes, and once it reads 205°F / 96°C, it's ready to be taken out!