Fudgy, rich, and deeply chocolatey brownies with a subtle tang that enhances the chocolate flavor. Nobody will guess the secret ingredient.

Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving overhang for easy removal.
Melt the butter and chocolate chips together in a microwave (30-second intervals, stirring between) or in a double boiler. Let cool slightly.
Whisk the sugar into the melted chocolate mixture. Add the sourdough discard, eggs, and vanilla. Whisk vigorously for about 1 minute — this builds the shiny crackly top.
Sift in the flour, cocoa powder, and salt. Fold gently until just combined.
Pour batter into the prepared pan and spread evenly. Sprinkle with flaky salt if using.
Bake for 25-28 minutes. A toothpick should come out with moist crumbs (not wet batter). Don't overbake!
Let cool completely in the pan before cutting. For the cleanest cuts, refrigerate for 1 hour first.