In a 12-ounce or larger microwave-safe mug, whisk together the flour, sugar, cocoa powder, baking powder, and salt with a fork until no lumps remain.
Add the milk, oil, and vanilla extract to the dry ingredients. Stir gently with the fork just until the batter comes together and no dry flour is visible, about 10-15 strokes.
Drop the Nutella in small spoonfuls on top of the batter. Use a knife or skewer to gently swirl it through the batter in a figure-eight motion, creating ribbons without fully mixing it in.
Microwave on high power for 60-90 seconds, checking at 60 seconds. The cake is done when the top looks set and springs back lightly when touched. It will rise up significantly during cooking.
Let the mug sit for 30 seconds to finish cooking from residual heat. The cake will deflate slightly and firm up. Top with extra Nutella, whipped cream, or ice cream if desired, then enjoy straight from the mug.
