Line muffin tins with liners and preheat the oven to 350°F.
In a stand mixer or large bowl, combine melted butter, sugar, and vanilla. Add eggs and mix well.
Incorporate flour, cocoa powder, coffee (or water), baking soda, and salt. Mix until fully combined.
Fill each muffin tin 2/3 full and bake for 25-28 minutes. Allow to cool completely.