First brown the butter. In a small, deep pot over medium-high heat, cook the butter until it’s a deep honey color, whisking frequently to scrape the bottom of the pot, and little brown bits form on the bottom of the pot, about 4 minutes. Turn off the heat and allow it to cool in the pot until the solids are a deep brown, about one more minute, taking care that they don't burn, then transfer to a bowl to cool completely and solidify at room temperature. This usually takes about 4 hours, so feel free to brown the butter ahead of time. The brown butter can also be transferred to a heat proof bowl and placed in the fridge for about an hour. 

If chilling, remove the brown butter from the fridge and let it soften at room temperature- it should be the same consistency as the regular butter. In a mixing bowl cream together the butter and brown butter with a wooden spoon, hand beaters or a stand mixer with the paddle attachment. The mixture should be uniform and smooth. Add the sourdough starter, vanilla and powdered sugar and mix until fully combined, making sure there’s no lumps of powdered sugar.

Add the rosemary, all-purpose flour and kosher salt. Gently fold together until mostly combined, and then add the hazelnuts and white chocolate chunk, mixing to fully incorporate.

Cover your work surface with a large piece of plastic wrap and turn the dough out onto it. Using your hands, form the dough into a log about 2 1/2” in diameter and 12” long. You can dust your hands with a little flour if it’s sticky. The log doesn’t have to be perfect. Wrap it in the plastic and roll it back and forth a few times on your work surface to smooth into a cylinder. Transfer to the fridge to chill for at least 2 hours or overnight. Alternatively, the dough can be frozen for up to two weeks. Thaw overnight in the fridge before using. 

When ready to bake, heat the oven to 325 degrees. Take the dough out of the fridge and let it come up to soften slightly so it slices with ease; about 15 minutes. Place about ¼ cup demerara sugar in a bowl. Unwrap the log of dough, trim off the ends, and cut off 1/2” thick slices. Roll the top and sides of each cookie in the sugar and place on a lined sheet tray, spaced about 2” apart. These cookies don’t spread out much when baked and you can fit 8 to 10 cookies on a baking sheet.

Transfer to the oven and bake until just golden on the edges but still pale in the centers, about 15 to 20 minutes. Allow to cool on the sheet pans before enjoying.