Line a quarter baking sheet with wax paper. Use a quarter teaspoon measuring spoon to scoop out little balls of dulce de leche. Place them on the baking sheet. Place baking sheet in freezer while you prepare cookie dough. 

In a medium sized mixing bowl, whisk to combine the flour, baking soda, cinnamon, salt and rolled oats. Set aside. 

In a large mixing bowl, use an electric mixer fitted with the paddle attachment to beat the butter and sugars on medium high for about 2 minutes until fluffy. 

Scrape down the sides of the bowl. Add the vanilla extract and the egg and beat at medium speed for 1 minute.

Add the sweetened condensed milk. Beat until combined. Add the dry ingredients. Beat on low til combined. Add the chocolate chips beat until just combined. 

Use a 1 tablespoon cookie scoop to scoop out cookie dough balls. Place dough balls on a baking sheet. Cover with plastic wrap. Chill in the fridge for 1 hour. 

Take out cookie dough balls. Use your thumb or a rounded 1/2 teaspoon measuring spoon to create an indentation in half the cookie dough balls. Take dulce de leche out of the freezer. Place one ball of dulce de leche in half the cookie dough balls. Place empty cookie dough half over the top of the dulce de leche half. Pinch the sides together to seal the dulce de leche inside. Roll cookies gently in your hand to give them an evenly round cookie dough ball shape. Place cookie dough balls on baking sheet. Cover with plastic wrap. Chill 3o minutes. 

Preheat oven to 375 Fahrenheit. Bake cookies two inches apart for 10-12 minutes until lightly golden on top. Allow to cool on baking sheet five minutes then transfer to a cooling rack to cool. Sprinkle with flaky sea salt, if desired.