Preheat your oven to 350°F (175°C) and grease a mini muffin tin.
In a medium saucepan, melt the butter over low heat.
Remove from heat and stir in the sugar, eggs, and vanilla until well combined.
Add the cocoa powder, flour, salt, and baking powder, and mix until just combined.
If using, fold in the chocolate chips.
Pour the batter into the prepared mini muffin tin, filling each cup about 2/3 full.
Bake for 10-12 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cool in the pan for a few minutes before transferring to a wire rack to cool completely.