Melt butter in a small bowl in the microwave and let cool slightly.
Mix flour, sugar, cocoa powder, baking powder, and salt in a microwave-safe mug.
Stir in milk, water, vanilla extract, and the melted butter.
Stir in chocolate chips.
Microwave for 80 seconds on high.
Let cool for a minute or two.
Top with whipped cream and more chocolate chips, if desired.
Notes
Expert Tips:
Use a standard-size mug (at least 10–12 oz) with straight sides: this makes it easier for the cake to rise.
Don’t overmix the batter—stir just until combined. Overmixing makes for a more rubbery texture.
Microwave power can vary. If the top has very small patches that still look at little wet, that is fine—it will cook a little bit more after you remove from the microwave. But if your cake still looks wet in the center, microwave in 5–10 second bursts until done.
Scrape the cake batter down the sides of the mug before baking so you don’t get any hard overcooked spots on the edges.
Wait a minute or two before digging in—these cakes get hot!