In a large bowl, whisk together the milk, egg, melted butter, and vanilla until combined.
In a separate small bowl, whisk together the flour, cocoa powder, sugar, and baking powder.
Add the dry ingredients to the wet ingredients and stir gently until the mixture is combined and no large dry streaks remain; the batter should be slightly lumpy—do not overmix.
Fold the mini chocolate chips into the batter gently until evenly distributed.
Heat a griddle or large nonstick skillet over medium heat. Lightly grease the surface with a little of the melted butter from the ingredient amount (use a brush or a folded paper towel).
Using a 1/4-cup measure, scoop batter onto the hot griddle, leaving space between pancakes to avoid overcrowding.
Cook the pancakes 2–3 minutes on the first side, until bubbles form around the edges and the top begins to set. Flip and cook 1–2 minutes more, until the pancakes are set and cooked through.
If keeping pancakes warm while you finish the batch, transfer them to a baking sheet and hold in a 200°F oven for up to 30 minutes.
Serve immediately, optionally sprinkling with additional mini chocolate chips from the ingredients.
Store any leftover pancakes in an airtight container in the refrigerator for up to three days.