Cream the Butter and Sugar: In a large mixing bowl, cream together 1 cup of softened unsalted butter and 1 cup of granulated sugar using an electric mixer on medium speed for about 4 minutes, until the mixture is light and fluffy.

Add Egg and Vanilla: Crack in 1 large egg and pour in 1 teaspoon of vanilla extract, mixing on low speed until everything is just combined.

Combine Dry Ingredients: In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking soda, and ½ teaspoon of salt. Gradually add this to the wet ingredients until a soft dough forms.

Chill the Dough: Cover the dough with plastic wrap and chill it in the refrigerator for 10 to 15 minutes.

Prepare Cinnamon-Sugar Coating: Mix together ¼ cup of granulated sugar with 1 tablespoon of ground cinnamon in a shallow bowl.

Shape and Coat the Cookies: Preheat your oven to 350°F (175°C). Scoop the dough into tablespoon-sized balls, roll in cinnamon-sugar, and place on parchment-lined baking sheets.

Bake the Cookies: Bake the cookies for 9 to 11 minutes, or until they are lightly golden around the edges yet still soft in the center.

Cool the Cookies: Remove the cookies from the oven and let them cool on the baking sheets for about 5 minutes before transferring them to wire racks.

Prepare the Filling: Melt 2 tablespoons of butter in a small saucepan, then stir in 1 cup of dulce de leche until smooth.

Assemble the Sandwich Cookies: Spread a generous dollop of the filling on the bottom of half of the cookies and top with the remaining cookies.