In a large mixing bowl, cream together the softened butter and 1 cup of granulated sugar for about 4 minutes until light and fluffy.

Add the egg and vanilla extract; continue mixing for another minute.

Gradually stir in the all-purpose flour, baking soda, and salt until a dough forms.

Chill the dough in the refrigerator for 10-15 minutes.

Combine 1/2 cup of sugar and 2 teaspoons of ground cinnamon in a shallow bowl.

Roll the chilled dough into 16 to 24 balls and coat them in the cinnamon-sugar mixture.

Place the coated balls onto a baking sheet, spaced apart.

Bake in a preheated oven at 350°F for 9 to 11 minutes until the edges are golden and the centers are soft.

Melt 1 tablespoon of butter in a saucepan, then stir in the dulce de leche until smooth.

Once the cookies have cooled, spread a layer of dulce de leche on one cookie and top it with another to create a sandwich.

Let them rest for about an hour before serving.