• Preheat the oven to 325F then line a baking sheet with aprchement paper and set aside.
• Cream the butter, granulated sugar, and brown sugar together until light and fluffy. Add in the egg, and vanilla extract and mix until combined.
  1 stick butter, 3/4 cup granulated sugar, 1/4 cup brown sugar, 1 large egg, 1 tsp vanilla extract
• Gently mix in the flour, baking soda, cream of tartar, salt, and cinnamon until the flour mixture disappears. don't overmix so that your cookies stay soft and chewy. The cookie dough will be slightly crumbly but don't worry once you top the cookie dough with the sweetened condensed milk will soften it.
  2 & 1/3 cup all purpose flour, 3/4 tsp baking soda, 1/4 tsp cream of tartar, 1/4 tsp salt, 1 tsp cinnamon
• Scoop out 8 equally sized cookie dough balls and place them on a baking sheet, gently press down to about 2/3 of an inch in thickness. Make sure to leave about 2 inches between each cookie dough ball so they don't stick together.
• Drizzle each cookie dough with about 1/2-1 tsp sweetened condensed milk, and bake in the preheated oven for 13 minutes. Then let them cool down fully on the hot baking sheet to allow them to keep baking in the center. Cool down fully before frosting.
  3 tbsp sweetened condensed milk

Make the cinnamon buttercream

• Cream the butter, sugar ,vanilla extract, and cinnamon until smooth and fluffy.
  4 tbsp butter, 1/2 cup powdered sugar, 1/4 tsp cinnamon, 1/4 tsp vanilla extract
• Top each cooled-down cookie with about 1/2-1 tablespoon of cinnamon buttercream and enjoy.