Preheat oven to 350 degrees. Prepare a baking sheet with a silicone baking mat or parchment paper. Set aside.
In a mixing bowl, use a hand-held mixer to blend together the butter and sugar until light and fluffy.
Add the egg and vanilla extract. Beat until well mixed into the butter and sugar.
Add in the flour, baking soda, salt, and cinnamon. Mix until well incorporated.
Portion cookie dough into 1-tablespoon portions. Roll into balls and place on the baking sheet - at least 2 inches apart. Bake one batch at a time on the center rack for 10 minutes.
While the cookies are baking, whisk together the 1 cup of sugar and 2 teaspoons of cinnamon. Set aside.
Remove cookies from oven and let sit for less than one minute. Use a slotted metal spatula to lift the cookie. Gently place the cookie into the cinnamon sugar mixture and coat the cookie well. Transfer cookie to a wire cooling rack to cool for 1 hour.
Once all cookies are baked and cooled, begin the assembly. Add about 2 teaspoons of the dulce de leche to the bottom of a cookie. Place another cookie - top side up - on top of the dulce de leche and press gently to create a sandwich. Don't press so hard that the caramel oozes out.
Store in the refrigerator until ready to eat.
The prep time includes the after baking assembly as well.The dulce de leche will continue to soften if left at room temperature. To serve, remove the desired number of cookies from the refrigerator and set aside for five to ten minutes before serving.