Melt the butter in a microwave safe mug for around 10-20 seconds.
Mix in the sugars, vanilla and salt with the melted butter. Stir in the egg yolk until fully combined.
Add in the flour and cocoa powder, and mix until fully incorporated. The batter should be thick. Fold in the chocolate chips.
Place in the microwave, and microwave for around 45 seconds.
Notes
Allow the brownie to cool. This allows the texture to firm up more, as the brownie continues cooking after taking it out of the microwave.
Swap out the chocolate chips. Use your favorite mix-ins! My favorite swap is either M&Ms or chopped walnuts.
Make sure you use a standard coffee mug. A standard 10-12 oz mug works best. Avoid using bowls or wide bottomed coffee mugs, since the batter will spread out thin on the bottom and get overcooked.
Adjust the sweetness. Feel free to slightly reduce the sugar if you prefer a less sweet brownie, especially since the chocolate chips add extra sweetness.