Preheat oven to 350. Line two cookie sheets with parchment paper or silpats.

Stir together the flour, nutmeg, baking soda, and salt. Set aside.

Cream the butter and brown sugar until completely combined and smooth. Beat in the pumpkin and sour cream, vanilla, and egg until well mixed. Scrape down the sides and bottom of the bowl as needed.

On low speed, add the flour mixture in 3 additions, mixing just until combined.

Scoop the dough onto the prepared sheets with about 2 tablespoons of dough. Use a spoon to press down lightly on the tops of the cookies to flatten them just a bit. Bake for 13-15 minutes, or until the centers are baked through. You can check this with a toothpick like a cake.

Remove to a wire cooling rack and let cool completely.

Beat the butter at medium-high speed until smooth, 1-2 minutes. Add in the vanilla extract and pinch of salt. Beat mixture at medium-high speed to combine, about 15 seconds.

Slowly add powdered sugar until incorporated. Add 2 Tbsp. of milk and beat until incorporated. Add the cinnamon and beat for another minute.

Use an offset spatula or butter knife to spread onto the cooled cookies.