Preheat oven to 350 degrees F. In a medium mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add granulated sugar, brown sugar, baking powder, and the 1/4 teaspoon salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour.
Shape dough into 1-inch balls. On an ungreased cookie sheet, arrange nuts in groups of three with groups 2 inches apart. Place each ball of dough on top of a nut cluster, with nuts peeking out from underneath ball. Flatten dough slightly.
Bake for 8 to 10 minutes or just until edges are firm. Cool on cookie sheet for 1 minute. While cookies are warm, spread each cookie with 1 teaspoon of the dulce de leche. If desired, sprinkle with sea salt. Transfer cookies to a wire rack; cool.