Cook butter in a medium saucepan over medium heat until the butter turns a deep golden brown color (about 5 minutes). Transfer to a bowl and let cool at room temperature until semi-solid.

Add hazelnuts, flour, sugar, salt and rosemary to the bowl of a stand mixer fitted with the paddle attachment. Mix on low until ingredients are evenly combined. Add brown butter in several additions while beating on medium speed until the dough looks like wet sand. The dough should hold together when pinched. If not, add a splash of cold water and mix again.

Turn the dough out on to a work surface and shape into 2 logs 1 1/2-inches in diameter. As you are shaping the logs, the dough will crumble and break. Just keep on pressing and rolling until it comes together. Wrap the logs in plastic wrap and freeze until hard (30 - 40 minutes). At this point you can keep the dough in the freezer for up to 3 days.

When ready to bake, preheat an oven to 350°F and place a rack in the middle position. Line 2 baking sheets with parchment paper.

Working with 1 log at a time, slice into 1/4-inch round cookies, and arrange on the prepared baking sheets. You should be able to fit around 18 per sheet. If a cookie crumbles as you are slicing (this can happen if your knife hits a bigger bit of hazelnuts), just press it back into a round shape. Working in batches, bake for 13 - 15 minutes until the cookies are lightly browned around the edges. Let cool for 5 minutes on the baking sheet, then transfer to a cooling rack.