Preheat the oven to 350°F (180°C). Prepare a baking sheet by buttering it, or line it with a Silpat mat or parchment paper.

Sift together the flour and baking powder. In a mixing bowl—using an electric mixer on low speed or mixing by hand—combine the butter, powdered sugar, and a pinch of salt until fully integrated and pale in color. (This takes about 3 minutes with a mixer.)

Add 1 whole egg and the yolk from the second egg, and continue beating on low speed until fully incorporated. Save the egg white.

Add the sifted flour and baking powder mixture, and blend into the batter by beating on low speed or working with a fork; you should end up with a soft, rather pliable dough.

Form 30 small balls of dough—roughly the size of a large marble, or about 1 ½ cm in diameter. Roll the dough balls first in the leftover egg white, and then in the chopped walnuts.

Place them on the prepared baking sheet and bake for 15–18 minutes, until the bottoms are golden brown and the tops are just beginning to turn golden.

Let cool on a wire rack and serve chilled.