Heat the oven to 325°F (160°C). Grease a 24-cavity mini muffin pan well, including the seams.

Whisk the warm melted butter and sugar together for 60–90 seconds.

Add the eggs one at a time, whisking well after each one. Add the vanilla with the second egg.

Keep whisking for 60–90 seconds more, until the batter looks glossy and starts pulling from the bowl sides in thin sheets.

Add the flour, cocoa powder, salt, and baking powder.

Fold gently with a spatula until no dry streaks remain.

Fold in the chocolate chips, if using.

Fill each cavity about 3/4 full.

Bake for 11 minutes for fudgy centers, or up to 13 minutes for more set edges.

Cool in the pan for 10 minutes, then loosen the edges and remove.