Start by browning the butter. You'll take ½ cup of butter and melt in a medium saucepan, continue to cook past the point of boiling. You will do this over medium heat, past the point of the butter foaming, until it turns a nice golden brown, and has a nutty smell. Make sure you are stirring and watching it, because it will turn from perfectly brown to burnt fairly quickly. Once finished browning, pour into a bowl and allow to cool completely. I usually put it in the freezer for about 15 minutes to speed up the process.
Next you'll make the cinnamon filling bits. In a medium size bowl cream together 2½ tablespoons of softened butter, brown sugar, cinnamon, flour and pinch of salt. Scoop into about ½ teaspoon size balls (very small), place on some wax or parchment paper and freeze about 15 minutes while making the cookie dough.
Preheat oven to 350 F. Line a baking sheet with parchment paper.
Place the butter and sugar in a stand mixer fitted with a paddle attachment. Beat on medium-high speed until light and fluffy, about 3-4 minutes. Add egg and mix on low until smooth, about 30 seconds. Beat in vanilla.
Slowly mix in the flour, baking powder, baking soda, salt and nutmeg, being careful not to over mix.
Take the cinnamon bits out of the freezer and gently fold into the dough. I like to keep 10 of them out to place one on top of each cookie before baking.
Scoop the dough into large balls, I like to use a 3 tablespoon cookie scoop for these. Push one cinnamon ball into the top of each cookie dough ball.
Place 5-6 cookie dough balls on your lined baking sheet. Bake the cookies for 10-13 minutes. You're looking for a light golden brown around the edges. The cookies might spread out in some spots because of the buttery cinnamon filling. If they get misshapen just run a spatula or spoon around the cookies right when they come out of the oven.
Mix together the glaze and drizzle or brush over the cooled cookies and allow the glaze to harden.