Adjust oven rack to upper-middle position and heat oven to 400˚F degrees. Spray a 9-inch non-stick cake pan with baking spray and line with parchment paper. Wrap a piece of aluminum foil around the outside of the pan (this will prevent over browning). Set aside.
Combine brown sugar, granulated sugar and cinnamon in small bowl. Add the melted butter (1 tablespoon) and stir with a fork until mixture resembles wet sand; set filling mixture aside.
Whisk together flour, sugar, baking powder, baking soda, and salt in large bowl.
Brush rectangle with 2 tablespoons of melted butter. Sprinkle evenly with filling, leaving a 1/2-inch border of plain dough around edges. Press filling firmly into dough. Loosen dough from work surface at one long side (use a bench scraper or metal spatula if needed to get it started). Roll dough, pressing lightly, to form a tight log. Pinch seam to seal. Place log seam-side down and cut evenly into eight pieces. With hand, slightly flatten each piece of dough to seal open edges and keep filling in place.
Place one roll in center of the prepared nonstick pan, then place remaining seven rolls around the perimeter of the pan. Brush tops with the 1 tablespoon remaining butter.
Cover and place in refrigerator for up to 12 hours (or you can bake right away). The next day, bake until edges are golden brown, 23 to 25 minutes. Cool for 10 minutes. Use a small metal knife, if needed, to loosen buns from sides of the pan. Place a dinner size plate over the pan and invert rolls onto a plate then invert again onto a cooling rack.
11. While buns are cooling, line rimmed baking sheet with foil or parchment paper (for easy cleanup) and set rack with buns over a baking sheet. Whisk powdered sugar, butter (1 tablespoon), vanilla and water together in a small bowl until smooth. Add more water if needed to make a thick but pourable icing. Drizzle icing evenly over buns and serve warm.
See Café Tips above for further instructions.
There is a total of 8 tablespoons melted butter in this recipe, including the icing. I like to melt it all at once to make things easier.