In a small bowl, whisk together the all-purpose flour, brown sugar, and cocoa powder until evenly combined.
Add the milk, vegetable oil, a pinch of salt, and a splash of vanilla (and coffee extract if using). Stir until a smooth batter forms.
Lightly spray a microwave-safe mug with nonstick cooking spray and pour the batter into the mug.
Microwave on high for 1 to 1½ minutes, watching for the top to set but remain moist; avoid overcooking.
Let the brownie cool for a minute, then top with sweetened whipped cream if desired and serve directly from the mug or turned out onto a plate.