Place the butter into a saucepan and over medium/high heat cook until the butter melts, bubbles and then foams. Keep a close eye on it as it can burn quickly, when the milk solids have browned the water will have been evaporated off so remove from the heat and set aside for 30 minutes or so, to cool slightly. Once browned you should have 185g unsalted butter left (thats if using butter with an 82% fat content).
Whilst the butter is browning place the flour, baking powder, baking soda and salt into a large bowl and whisk together to combine.
When ready to make the cookies place the butter and sugars into a large bowl and using an electric mixer, fitted with the whisk attachment, whisk for a couple minutes to combine and to break up any lumps. Add the egg yolks and whisk for 2-3 minutes on medium/high. Place the bowl on your scale and measure in the required sourdough discard, adding the vanilla as well. Mix in for a few minutes or until the mixture becomes smooth and fully combined, it should look a little like a thick cake batter. Add in the flour mixture and mix in on low speed, just until everything comes together as a dough. Finally, switch to the paddle attachment and add the chocolate, mixing briefly until evenly distributed. Press a sheet of clingfilm onto the surface of the cookie dough and refrigerate for at least 4 hours before baking (my preferred time frame to bake these is between 4-24 hours).
Bake: 16min at 350F.