Whisk dry ingredients. In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
Cream butter and sugar. In a second mixing bowl, with an electric mixer cream together butter and sugar until light and fluffy, about 2 minutes.
Add egg and vanilla. Add egg and mix just until combined. Scrape the sides and bottom of the bowl as needed. Add vanilla extract and combine.
Add in dry ingredients. Add the flour mixture, and mix at low to medium speed until mixture is evenly moistened.
Chill dough. Place dough onto a large piece of plastic wrap and wrap up and flatten into a large disc of dough. Chill for 2 hours.
Make filling. In a small bowl, combine brown sugar and cinnamon.
Roll out cookies. Divide dough in half. Roll out cookie dough into a 9 by 11 inch rectangle to ¼-inch thickness.I find it easier to roll out between two sheets of parchment paper so the dough doesn’t stick and it’s easier to roll up after. Spread half of the melted butter on dough. Sprinkle half of filling evenly over doughleaving a ½-inch border along one long edge. Roll up dough tightly into a log (the tighter the roll the better so you get a neat spiral). Repeat with second half of dough. Wrap in plastic wrap and chill dough for 2 hours.
Preheat and prepare sheets. Preheat oven to 400oF. Prepare cookie sheets with parchment paper.
Cut cookies. Remove dough from fridge and unwrap. Keep the second dough in the fridge. Cut into ½-inch thick slices and place on baking sheet 2 inches apart.
Bake and cool cookies. Bake cookies for 10 to 12 minutes, until edges are just slightly browned. Remove from the oven and cool on the pan for 5 minutes, before transferring to a rack to finish cooling completely.
Make icing. In a small bowl, whisk together icing ingredients. Drizzle over each cooled cookie.