Place flour, sugar, cocoa, baking powder and salt in a microwave-safe ramekin or mug. Stir well with a fork.
Add the oil and milk and stir until just combined.
Add the hot water. Stir until fully incorporated. The texture of the batter should be consistent throughout. It will be thin, but shouldn’t have any watery spots.
Microwave for about 35-40 seconds, depending on your microwave. The cake should rise and look just barely set on top. The edges may even look watery/wet still, but that's ok because of carryover cooking, plus there's no eggs to worry about. OPTIONAL: Top immediately with chocolate chips and let it rest for 1 minute before digging in.
Notes
Don’t Overcook: Your chocolate mug cake should be soft and moist, not dry. My 1200W microwave takes 35 seconds for a slightly moist and fudgy consistency, whereas 40 seconds will create a fluffier cake. But it changes that fast! Start with 30 seconds and add more time in 5-second intervals if needed. Keep in mind carryover cooking time.
Ramekin vs Mug: I prefer a 4x2-inch (6oz) ramekin* for a wide and even baking surface. An 8oz mug* will still do, however, it will only fill about half of the mug. (Use a bigger mug if doubling the recipe and adjust the cook time as needed.) *affiliate links*
Toppings and Customizations:
Top with chocolate chips (mini or regular size)
Swirl batter with slightly melted peanut butter or Nutella, or dollop it firmly into the center for a more gooey, molten lava type cake
Top with a scoop of ice cream or dollop of whipped cream
Add fresh berries for a tart contrast that compliments the chocolate well