To make brown butter: In a small pot, melt the butter over medium heat, whisking constantly. After it fully melts, the butter will foam up and then start to turn golden brown. When the butter is golden brown, take it off the heat immediately and pour it into a small bowl. Let cool for 10 minutes.
In a large mixing bowl or the bowl of a stand mixer, mix brown butter, sugar, and brown sugar.
Add pumpkin puree, egg yolk, and vanilla. Mix until the egg is fully incorporated.
In a separate bowl, add the dry ingredients (flour, pumpkin pie spice, baking soda, cream of tartar, and salt). Stir to combine.
While mixing on low speed, add the dry ingredients to the wet ingredients. Mix until a dough forms. The dough should be soft but not sticky. Cover and chill for at least 1 hour in the fridge.
During the last 15 minutes of chilling time, preheat the oven to 350. In a small bowl, make the cinnamon sugar mixture (1/3 cup sugar + 2 teaspoons cinnamon).
Remove the dough from the fridge and roll into 1-inch balls. Roll in cinnamon sugar and place on a greased baking sheet.
Bake for 10 minutes. DO NOT OVER-BAKE. Let cool on the pan completely.
Once the cookies are cool, serve and enjoy!