Grab a small mixing bowl or cereal bowl. Choose your serving mug - a coffee mug or small ramekin works great. If you plan to flip the cake out later, spray your mug with cooking spray first.
Add both cocoa powders to your mixing bowl. Spoon in the flour without packing it down. Add salt, baking powder, and sweetener. Whisk everything together vigorously for 30 seconds to blend well and prevent lumps.
Pour in milk slowly while stirring. Drizzle in the oil (or substitute with melted coconut oil, applesauce, or mashed banana). Add vanilla extract last. Stir the batter until completely smooth with no flour streaks. The consistency should resemble thin brownie batter.
Pour batter into the prepared mug, filling no more than halfway full. Place mug in center of microwave and cook on high power for 30 seconds initially. Check the cake - it should look mostly set but slightly shiny on top. If needed, cook in additional 10-second bursts until done (about 50 seconds total). Let rest for 1 minute before eating.
Oven Alternative: Preheat oven to 350°F. Pour batter into oven-safe ramekin and bake for 12-14 minutes. Test doneness with a toothpick - it should come out with just a few moist crumbs.
Let cake cool for 2 minutes. Top with whipped cream, ice cream, chocolate chips, fresh berries, caramel sauce, or enjoy plain. Serve immediately for best texture.