Mix the dry ingredients together. This is the flour, sugar, baking powder, salt, and cocoa.
stir them together with a whisk (I actually prefer a butter knife because it gets along the sides better).
Pour in the melted coconut oil, milk, and vanilla. Stir as best as you can to break up the chunks and deposits at the bottom of the mug. Not very many lumps will remain.
Sprinkle your chocolate chips on top, do not stir. The more you add the more gooey your brownie will be, less will make it more cakelike.
Micrwoave for 40-60 seconds or bake in an oven at 350 degrees for 10 minutes. Make sure if doing the latter that you have an oven safe mug.
Notes
Sweetness: You can add up to 1 more tablespoon of sugar for a sweeter brownie (optional).Cocoa powder: To reduce the richness, use ½ tablespoon less cocoa powder; for a richer chocolate flavor, add 1 teaspoon instant coffee.Liquid: Use coffee creamer, cold coffee, or even water instead of milk.Oven instructions: If you want to bake the brownie in the oven, use an oven-safe mug on a baking sheet at 350°F for 10 minutes, then test for doneness.Troubleshooting: The brownie is done when a toothpick comes out clean. If it's chewy or rubbery, it may have been overcooked due to microwave wattage differences.Make ahead: Prepare the dry ingredients in advance and store them in a small bag or container for a quicker brownie-making process.