Lightly grease a microwave-safe mug with a wide opening and a capacity of at least 10–12 ounces. In a small bowl, combine the flour, baking powder, granulated sugar, and salt.

Stir in the milk and melted butter just until a soft dough forms. Avoid overmixing, since a heavily worked dough can produce a tougher, denser cinnamon roll.

Turn the dough onto a lightly floured surface and press or roll it into an approximately 4 by 6-inch rectangle. Spread the softened butter over the surface, then sprinkle evenly with the brown sugar and cinnamon.

Roll the dough up from the short side into a compact spiral. Place the roll seam-side down in the prepared mug so the coil stays together during cooking.

Microwave on high for 45–60 seconds, then check the center. If it is still wet, continue in 10-second intervals; most microwaves take about 60–75 seconds depending on wattage.

Let the cinnamon roll stand for 1 minute before serving. For the glaze, stir the confectioners’ sugar with the milk or water and optional vanilla, adding only enough extra liquid to make a drizzle.