Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Add the egg and vanilla extract to the butter mixture, mixing until fully combined.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined.
If using, fold in the chopped pecans or walnuts for an added crunch.
Scoop tablespoon-sized amounts of dough and roll them into balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes until the edges are lightly golden but the centers remain soft.
While the cookies cool, prepare the icing by whisking together powdered sugar and milk until smooth. Adjust with more milk if necessary for desired consistency.
Drizzle or spread the icing over the cooled cookies and allow it to set for a few minutes before serving.
Notes
Arrange the cookies on a decorative plate, dust with a little extra cinnamon, and garnish with a sprig of mint for a pop of color.