Heat treat your flour. Preheat oven to 350°F. Spread the flour onto a baking sheet and into a thin layer. Cook for 5 minutes, then let cool completely before using.
Cream together the softened butter and granulated sugar. Slowly pour in the sweetened condensed milk and mix until combined. Add in the vanilla extract and mix to combine.
In a bowl, mix the flour, cream of tartar, cinnamon, and salt together. Slowly add the flour mixture into the wet ingredients. Once everything is added, the dough should be a little wet and slightly sticky. Pop it in the refrigerator for 20-30 minutes to chill and firm up.
After chilling the dough, use a medium cookie scoop to scoop even balls of dough. Roll each scoop of dough into a ball.
Combine the 1/3 cup granulated sugar and 1 teaspoon of cinnamon in a small bowl. Roll the cookie dough balls in the cinnamon sugar mix until completely coated. Enjoy! Store in the refrigerator for 4-5 days.