In a medium-sized mixing bowl, whisk together the flour, cream of tartar, baking soda, ground cinnamon and salt until well combined. Set aside.

In a large mixing bowl using an electric mixer, cream together the butter, granulated sugar, and brown sugar for about 1-2 minutes or until well combined. Mix in the egg, egg yolk, and vanilla extract until well combined, stopping to scrape down the sides of the bowl as needed.

Add the dry ingredients to the wet ingredients and mix until just combined. Cover tightly and refrigerate for at least 30 minutes.

Preheat oven to 350°F. Line large baking sheets with parchment paper or silicone baking mats and set aside.

Make the caramel. Mix butter, light corn syrup, light brown sugar, and vanilla extract over stovetop on medium heat.  

Stir often until it mixture comes to a rolling boil. Add can of sweetened condensed milk and return to boil for 3-4 minutes. Remove from heat.

Remove dough from fridge and scoop out to make small round balls. Roll around and toss in cinnamon & sugar mixture and place on cookie sheet with room in between each one.

Use bottom of measuring spoon (or thumb) to make indentation in center of each ball.

Bake for 8-10 minutes. Pull out cookies and press down centers again with spoon and let cool or about 10 minutes.

Spoon about 1 teaspoon Salted Caramel Sauce in center of each cookie. Garnish with flaked salt, if desired.