In a small bowl, whisk together the flour, baking powder, and granulated sugar. Add the melted butter, milk, and vanilla; stir with a fork until a soft dough forms.

Turn the dough out onto a lightly floured surface. Pat it into a rectangle about 4 by 6 inches.

In another small microwave-safe bowl, combine the butter, brown sugar, and cinnamon. Microwave on high for 20 to 30 seconds, or until the butter is melted. Stir until smooth.

Spread the filling evenly over the dough rectangle, leaving a small border along one long edge. Roll the dough tightly from the opposite long edge to form a log. Pinch the seam to seal.

Lightly grease a standard 12-ounce mug with nonstick spray. Place the rolled dough into the mug.

Microwave on high for 90 seconds. If the top looks wet, continue microwaving in 10-second bursts until the dough appears dry and springs back when touched. A toothpick inserted into the center should come out clean.

In a small bowl, whisk the powdered sugar and milk until smooth and pourable. Adjust with a few extra drops of milk if too thick.

Pour the glaze over the warm cinnamon roll in the mug, or carefully invert the roll onto a plate and drizzle the glaze on top. Serve immediately.