In a large bowl, add ½ cup softened butter, ⅓ cup sugar, 2 Tbsp brown sugar. Cream together butters and sugars until light and fluffy.
Add the 7oz sweetened condensed milk, 1 egg yolk, and vanilla. Mix everything until combined.
Add the 2 cups flour, 1 Tbsp cornstarch, 1 tsp baking powder, ½ tsp baking soda, ¼ tsp cream of tartar, ¼ tsp salt. Mix together until incorporated.
Using a medium cookie scoop, scoop roughly 1½ - 2 Tbsp balls of dough. Roll the dough and place on a baking sheet lined with parchment or silicone. Press lightly on the top to flatten the centers.
Bake the cookies for 6-8 minutes. Cookies should just barely be golden around the edges when removing them from the oven. Allow cookies to cool thoroughly.