Toast the Pecans:
Preheat the oven to 300°F (150°C).
Spread the pecans on a lined baking sheet and bake for 8–10 minutes until toasted. Let cool, then chop the pecans. Use 3/4 cup (90g) for the cookie dough and save the rest for garnish.
Make the Cookie Dough:
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the softened butter and brown sugar on high speed until smooth and creamy, about 2 minutes.
Add the egg and vanilla extract, and beat on high speed until combined, about 1 minute. Scrape down the sides of the bowl as needed.
Add the dry ingredients to the wet ingredients and mix on low speed until combined. Stir in the toasted pecans. If the dough seems too soft or sticky for rolling, add 1 more tablespoon of flour.
Divide the dough into 2 equal parts. Place each portion onto a piece of lightly floured parchment paper or a lightly floured silicone baking mat. Roll the dough out to about 1/4-inch thickness.
Lightly dust one of the rolled-out doughs with flour. Place a piece of parchment on top, then place the second rolled-out dough on top. Cover with plastic wrap and refrigerate for at least 1–2 hours or up to 2 days.
Bake the Cookies:
Preheat the oven to 350°F (177°C). Line 2–3 large baking sheets with parchment paper or silicone baking mats.
Remove one piece of dough from the refrigerator and use cookie cutters to cut into shapes. Re-roll the remaining dough and continue cutting until all the dough is used. Repeat with the second piece of dough.
Arrange cookies on baking sheets about 3 inches apart and bake for 12–13 minutes until lightly browned around the edges. Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Make the Brown Butter Icing:
Slice the butter into pieces and melt in a light-colored skillet over medium heat, stirring constantly. After 5–8 minutes, the butter will begin to brown and take on a nutty aroma. Remove from heat and let cool for 5 minutes.
Whisk in the confectioners’ sugar, milk, vanilla extract, and a pinch of salt until smooth.
Decorate the Cookies:
Dip the tops of the cooled cookies into the brown butter icing, or wait for the icing to thicken at room temperature (about 10–15 minutes) and spread it on with a knife or spatula.
Garnish with the remaining chopped toasted pecans.
Let the icing set at room temperature for a few hours before stacking or transporting the cookies.
Storage:
Store decorated cookies at room temperature for up to 3 days or refrigerate for up to 10 days.