Preheat oven to 350°F (176°C) and line a 9×9-inch baking dish with parchment paper.
In a saucepan over low heat, melt butter. Add chocolate chips, cocoa powder, and vanilla extract. Stir until smooth and fully melted. Let cool slightly.
In a large bowl, beat eggs, egg yolk, brown sugar, and granulated sugar for 7–10 minutes until light and creamy.
Add sourdough discard and the cooled chocolate mixture. Mix on low speed until combined.
Gently fold in flour and salt until just incorporated. The batter will be thick.
Pour batter into prepared pan and smooth the top.
Bake Easy Sourdough Discard Brownies for 35–40 minutes, until a toothpick inserted shows moist crumbs.
Cool completely in the pan before slicing into squares.
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