Preheat oven to 375°F.

Sift together flour, baking powder, and salt. Beat together butter, brown sugar, and vanilla with an electric mixer until pale and fluffy. Mix in flour mixture at low speed, then stir in walnuts.

Form 2 teaspoons dough into an egglike shape the size of an acorn. Make more “acorns,” arranging them 1 inch apart on ungreased baking sheets. Bake in batches in middle of oven until undersides are light brown, about 10 minutes. Transfer to racks to cool.

Dip half of each cookie in melted chocolate and then in chopped nuts. Place as coated on a baking sheet lined with wax paper to set, about 15 minutes.