Preheat your oven to 350 degrees Fahrenheit. Make your butter: Melt a stick of butter until just melted in the microwave. Allow to sit on the countertop until it returns to room temperature.

(Bowl #1) In a medium bowl combine brown sugar, eggs, vanilla, and the room temperature melted butter.

(Bowl #2) In a separate bowl combine flour, salt, baking soda, baking powder, nutmeg, and cinnamon.

Combine bowl #1 into bowl #2. Mix until evenly combined. You may have to use your clean hands! Place this in the fridge.

Now, make the Brown Sugar Filling. Combine butter, brown sugar, flour, and cinnamon. Place in fridge.

Use a cookie scoop to grab a portion of dough. Roll the dough in your hands to form a sphere then use your palm to slightly flatten it. Then, hold the cookie in one hand while slowly turning and pressing your thumb into the center. If this is too difficult read the section above title 'How To Make The Thumbprint Center'. Repeat until pan is filled.

Now, grab out your Brown Sugar Filling from the fridge. Grab a small amount and round it in the palm of your hands. Fit it into the center of the cookie. Repeat until cookies are filled.

Bake in oven for 13-15 minutes. Immediately remove from pan and place on a cooling rack when done.

Now, make the icing. Combine powdered sugar and milk. If you want to adjust this read the about section entitled, 'Brown Sugar Thumbprint Cookie Icing'.

Allow the cookies to cool then use a spoon to drizzle your icing on the cookies!