In a large bowl, whisk together the flour, baking powder, sugar, cinnamon and salt.
In a small saucepan set over low heat, combine the milk and butter. Cook just until the butter is melted. (Alternately, microwave the milk and butter together. See Kelly's Notes.)
Add the milk mixture to the flour, and using a spatula, mix just until combined.
Flour your work surface then turn out the dough and knead it for 1 minute until it comes together in a smooth ball. If the dough feels overly sticky, add more flour as needed, but do not over-knead the dough.
Cover the dough loosely with a damp towel and let it rest for 15 minutes.
Preheat the oven to 350°F. Grease an 8- or 9-inch baking dish with butter.
Lightly flour your work surface then roll the dough into a 10x14-inch rectangle.
In a small bowl, whisk together the brown sugar and cinnamon.
Brush the dough all over with the melted butter then sprinkle the cinnamon-sugar mixture on top.
Beginning at the long end, tightly roll the dough up into a log. Trim off the edges then slice the log into 9 rolls.
Arrange the rolls in the greased baking dish.
Bake the rolls for 25 to 30 minutes until they are pale golden and baked through. While the rolls are baking, make the frosting.