Fudgy Sourdough Brownies

Preheat the oven to 325ºF (163ºC), line an 8" square baking pan with parchment paper, and grease with oil or butter.
Cube the butter and add to a heat-proof bowl with the chocolate chips. Melt over simmering water, stirring with a spatula constantly, and set aside.
170 g Unsalted Butter, 168 g Semi-sweet Chocolate Chips
In a large mixing bowl, beat the sugars and eggs together for 4 minutes until frothy and pale yellow.
200 g Granulated Sugar, 100 g Light Brown Sugar, 2 Eggs, 1 Egg Yolk
Whisk in the cocoa powder, optional espresso powder, and vanilla until incorporated and whisk the sourdough discard in last.
Then, slowly whisk in the butter and melted chocolate mixture until incorporated.
40 g Dutch-process Cocoa Powder, 2 tsp Espresso Powder, 2 tsp Vanilla Extract, 120 g Sourdough Discard
Add the all-purpose flour and salt and fold in with a spatula until no dry spots remain.
75 g All-purpose flour, 1 tsp Kosher Salt
Pour the sourdough brownie batter into the prepared baking pan and bake for 35-40 minutes. The brownies are done when they puff up, a toothpick inserted in the middle has a few crumbs remaining, and they feel firm to the touch with a little give in the middle. They will continue to bake some while cooling.
Cool completely on a wire rack for at least an hour before slicing.
For clean slices, freeze the brownies for 20 minutes. Then, run a sharp knife under hot water, slice, clean the knife between slices, and repeat.