Preheat oven to 350 degrees F (175 degrees C).
Prepare a cupcake/muffin tin by lining it with paper or silicone liners, or buttering/spraying it to make sure nothing sticks.
Add eggs and sugar to the bowl of a stand mixer or a mixing bowl. Beat eggs and granulated sugar together until light in color and doubled or tripled in size. Use a stand mixer with a whisk attachment, or an electric hand mixer – it will be more difficult to whisk by hand, but it can be done!
Add in vanilla extract and mix to combine.
Combine the dry ingredients in a bowl: flour, cocoa powder, baking powder, and salt.
Combine oil, coffee, and milk in a separate bowl or container.
Alternate mixing in the dry and wet ingredients to the batter, adding in half the ingredients each time. Be sure to sift in the dry ingredients to avoid lumps. Mix on low speed between additions.
Once the batter is combined, pour it into the cupcake tin. Fill each cupcake up about halfway, and if you have leftover batter at the end, distribute it evenly among them. This is the best way to avoid cupcakes that are too domed.
Bake for about 15-20 minutes, until a knife inserted in the center of a cupcake comes out clean. It can be hard to tell when chocolate cake is done, but try not to overbake or the cake may be dry! Definitely check the cupcakes after 15 minutes since ovens can vary greatly.