Brown butter: Melt butter in a small saucepan over medium-low heat,. Once melted, continue to heat butter for 2 to 3 minutes swirling the pan occasionally to loosen the milk solids from the bottom of the pan. Remove from heat when the milk solids are golden brown and the butter has a nutty aroma. Transfer the butter to a heatproof bowl. Whisk the miso and powdered milk into the brown butter, if using. Let the brown butter cool for 10 minutes.
113 g butter, 21 g miso, 5 g milk powder
Make the cookie dough: Add brown sugar, granulated sugar, oil, egg yolks and vanilla to browned butter and whisk until combined. Let the mixture rest for 5 minutes.
20 g olive oil, 2 egg yolks, 100 g Imperial Sugar Dark Brown Sugar, 67 g Imperial Sugar Extra Fine Granulated Sugar, 8 g vanilla
Whisk the mixture after the rest period until it looks fluffy and lighter in color. Stir in sourdough discard until no traces of discard remain.
56 g sourdough discard
Fold the all-purpose flour, baking powder and salt into the brown butter mixture until no traces of dry flour remain.
180 g all-purpose flour, 10 g baking powder, 5 g salt
Add mix-ins: Stir in the chopped chocolate chips and walnuts (if using).
113 g dark chocolate, 100 g walnuts
Chill dough: Cover the mixing bowl with plastic wrap and refrigerate for at least 1 hour or up to 2 days.
Prepare for baking: As your dough chills, preheat the oven to 375 F. Line two baking sheets with parchment paper. Set aside.
Portion out cookie dough: Using a 3-tablespoon cookie scoop, divide the dough into 12 roughly equal portions. Arrange 6 cookies on each baking sheet, spacing them out evenly.
Bake: Bake cookies one tray at a time (refrigerate the remaining cookie dough portions) for 11 minutes or until golden brown. Remove the tray from the oven and let the cookies set for 5 to 10 minutes on the baking tray. Transfer cookies to a wire rack to cool.
Enjoy and store: These cookies are better after they have cooled slightly and the interior has set. Store any leftovers in an airtight container at room temperature for up to 4 days.