If you'd like to make the dough ahead and freeze for longer than a few days, put the wrapped dough in a sealed freezer bag or other container to avoid freezer burn.
1 cup unsalted butter
1 cup walnut halves, toasted and chopped small (none larger than pea-sized)
1 tablespoon rosemary, minced
2 cups flour
1/2 teaspoon salt
1/2 cup granulated sugar
1/4 cup confectioner sugar (or another 1/4 cup granulated, if you don't have confectioners)
Melt the butter in a medium sauce-pan or skillet over medium heat. Swirl occasionally and cook until all foaming has subsided, the butter is dark tan colored with little black specks, and smells fantastically nutty. Transfer to a heat-proof bowl and let it cool to room temperature. (The butter should be solid but still smooshy)
Mix the walnuts, rosemary, and all the dry ingredients together in the bowl of a standing mixer fitted with a paddle attachment (or medium-sized bowl with a hand mixer). While beating on medium-speed, add the cooled brown butter in several additions until it's completely incorporated and the dough looks grainy. The dough should hold together when you press some in your fist - if not, add a few teaspoons of cold water until it does.
Turn the dough out onto your work surface and shape it into two thick logs about 1 1/2 - 2 inches in diameter (or smaller if you want smaller cookies). The dough will be pretty crumbly, but that's ok. Wrap the logs tightly in plastic wrap and freeze until hard (30 minutes, or up to three days).
When ready to bake, heat the oven to 350°F. Prepare two baking sheets with parchment paper.
Take out and unwrap one of the logs. Slice it into 1/4 inch round cookies using a sharp knife and transfer to the cookie sheet. Place cookies about a half inch apart - they won't spread much during baking. If any of the cookies crumble, just press the dough back together with your fingers and a bit of water.
Bake for 10-13 minutes, until lightly browned around the edges and very fragrant. Allow to cool 5 minutes on the baking sheet before transferring them to a cooling wrack.
Prepare and bake the other half of the cookie dough as above. Cookies keep in a sealed container for about two weeks.
(Images: Emma Christensen)