1. Make the dough: Whisk the flour, baking powder, and salt together. Set aside.
2. Using a hand mixer or a stand mixer fitted with paddle attachment, beat the butter for 1 minute on high speed until creamy. On medium-high speed, beat in the granulated sugar until completely creamed and smooth, about 2 minutes. Add the egg and vanilla extract and beat on high until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed.
3. Add the dry ingredients to the wet ingredients and mix on low until combined. If the dough still seems too soft, you can add 1 Tablespoon more flour until it is a better consistency for rolling.
4. Shape and fill: Divide the dough into 2 equal parts. Roll each portion out in a rectangle (about 9x7 inches) onto a floured silicone baking mat (or floured parchment paper) to about 1/4″ thickness. Spread 1 Tablespoon of melted butter onto each rectangle. Mix the granulated sugar and cinnamon together then sprinkle evenly over each.
5. Working slowly, tightly roll up each rectangle into a 9-inch log. If the dough is cracking at all, use your fingers to smooth it out. Some cracks are OK. Chill the logs of dough for at least 2 hours. What I like to do is carefully transfer the logs to a cutting board or plate and cover loosely with plastic wrap.
6. Preheat oven to 350°F (177°C). Line a large baking sheet with parchment paper or a silicone baking mat. (Always recommended for cookies.)
7. Remove dough logs from the refrigerator. Cut into 1/2 inch slices. Place slices onto baking sheets about 2 inches apart.
8. Bake for 10-11 minutes, until lightly browned on the sides. Remove from the oven, allow to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before icing.
9. Make the icing: Whisk all of the icing ingredients together. Drizzle over cookies. Cover and store cookies at room temperature for up to 5 days.

Make ahead tip: Baked cookies (with or without icing) freeze well up to 3 months. Thaw overnight in the refrigerator. You can chill the cookie dough for up to 2 days (step 5). You can also freeze the cookie dough before rolling for up to 3 months. Allow to thaw overnight in the refrigerator, then bring to room temperature for about 1 hour. Then roll and continue with the recipe as directed.