Preheat Oven:
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Make Cookie Dough:
In a large bowl, cream together butter and sugar until light and fluffy. Beat in the egg and vanilla extract.
Combine Dry Ingredients:
In a separate bowl, whisk together flour, baking powder, and salt. Gradually add to the butter mixture and mix until a soft dough forms.
Shape Cookies:
Roll dough into 1-inch balls and place on prepared baking sheets, spacing about 2 inches apart. Flatten slightly with the bottom of a glass or your hand.
Bake Cookies:
Bake for 10–12 minutes, or until edges are lightly golden. Allow cookies to cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
Assemble Sandwich Cookies:
Spread about 1–2 teaspoons of dulce de leche on the flat side of one cookie, then top with another cookie to create a sandwich. Press gently to adhere.
Serve:
Enjoy immediately or chill slightly for firmer filling.