In one bowl, mix butter, sugars, egg, and vanilla with a mixer.

Add flour, baking powder, salt, and chopped walnuts. Mix until smooth.

To make cookies the same size, weigh each piece of dough on a kitchen scale.

You can make more small cookies or fewer large ones.

Flatten each piece, add 1 tsp dulce de leche in the center, seal, and roll into a ball.

Arrange on a lined baking sheet.

Preheat oven to 350°F (180°C).

Bake 10–12 minutes until edges are golden.

Cool 5 minutes on the tray, then transfer to a rack.