Prep: Preheat the oven to 325 F with a rack in the middle. Line an 8-inch baking pan with parchment paper.
Brown the butter: Melt the butter in a light-colored skillet over medium heat. Swirl the pan or scrape the bottom of the pan occasionally. Continue heating until foaming subsides and the milk solids appear darker and the butter smells nutty. Move to a large mixing bowl.
113 grams unsalted butter
Toast the walnuts: Transfer walnuts to a parchment-lined baking sheet. Toast for 5 to 8 minutes or until the nuts are fragrant.
68 grams walnuts
Make the batter: Stir the sugar into the hot melted butter. Add the egg in and whisk vigorously until the mixture looks thick, smooth and satiny. Stir in the vanilla and sourdough discard until fully combined.
200 grams dark brown sugar, 1 egg, 2 teaspoons vanilla extract, 56 grams sourdough discard
Fold in the salt, baking powder and flour until no traces of dry flour remain.
½ teaspoon salt, 5 grams baking powder, 160 grams all-purpose flour
Add in the chocolate chips and walnuts.
78 grams dark chocolate chips
Bake: Transfer the batter to your prepared pan. The batter should be very thick, almost like cookie dough. Flatten the batter out to the edges of the pan.
Bake for 35 to 40 minutes or until the center of the blondies bounce back when lightly touched and a toothpick inserted in the center comes out just a few moist crumbs.
Let the blondies set: Set the pan on a wire rack to cool for 30 minutes before slicing.
Enjoy and store: Slice the blondies and serve. Store leftover slices in an airtight container at room temperature for up to 4 days.