Place the butter in a medium saucepan. Cook over medium heat, stirring occasionally, until the butter melts and foams. Continue cooking until the foam subsides, brown specks form in the bottom of the pan, and the butter turns a nutty brown (8-10 minutes).

If you want to remove the browned bits, pour the butter through a strainer into the bowl of a stand mixer.* Beat on medium-low speed for 5-7 minutes to cool the butter to room temperature.

Whisk together the flour, cream of tartar, cinnamon, baking soda, and salt. Set aside.

Combine the eggs and milk in a small bowl. Whisk lightly.

Turn off the mixer when the butter has cooled to room temperature. Add the sugar and brown sugar. Beat until combined. Reduce mixer speed to low and pour in egg mixture. Beat until well-combined.

Add the flour mixture in three portions, beating just until combined after each addition.

Gather the dough into a mound in the bowl. Cover the bowl and refrigerate at least an hour, or up to 24 hours.

Preheat oven to 400°F. Line baking sheets with silicone liners or parchment paper.

Stir together the cinnamon-sugar ingredients in a wide-mouth bowl or shallow dish.

Using about 2 tablespoons of dough at a time, roll the dough into balls. Roll each ball in cinnamon-sugar to coat. Place the cookies about 1 & 1/2 inches apart on prepared baking sheets.

Bake one pan at a time for 10 minutes, or until cookies are cracked and set.**

Cool on the pans on wire racks for 10 minutes. Then, transfer the cookies directly onto wire racks to cool completely.