Step 1

Preheat the oven to 350 degrees F (175 degrees C). Place the walnuts on a rimmed sheet pan.

Step 2

Toast walnuts in the preheated oven until fragrant, 8 to 10 minutes. Transfer the walnuts to a plate and set aside.

Step 3

Whisk flour, baking powder, and salt together in a medium bowl.

Step 4

Add brown sugar and butter to the bowl of a stand mixer; cream on medium-low speed until combined. Increase speed to medium-high and beat until light and creamy, about 2 minutes. Reduce speed to medium-low and add the eggs, one at a time, mixing well after each addition and using a rubber spatula to scrape the bottom and sides of the bowl as needed.

Step 5

Stop the mixer, add the flour mixture, and mix on low until flour is nearly incorporated. Add walnuts and continue to mix until there aren't any streaks of flour in the cookie dough.

Step 6

Place about 1/2 of the dough in the center of a piece of parchment or waxed paper and use the paper to help you shape the dough into a 2-inch wide log. Roll the log back and forth so the sides are nicely rounded. Repeat with the remaining dough to make a second log. Set the parchment-wrapped logs on a baking sheet and freeze them until hard, about 2 hours.

Step 7

Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with parchment paper.

Step 8

Unwrap a log and place it on a cutting board. Use a sharp knife to slice the log crosswise into the thinnest possible rounds without having the cookies crumble apart, about 1/8- to 1/16-inch thick. Set the cookies 1 1/2 inches apart on the prepared baking sheet.

Step 9

Bake in the preheated oven until golden around the edges, 10 to 12 minutes. Remove from the oven and cool for 5 minutes before using a spatula to transfer the cookies to a wire rack.