Preheat oven to 350°F and line a 9" x 9" baking dish with parchment paper.
Melt butter in a small saucepan over low heat; add chocolate chips, vanilla extract, and cocoa powder.
Stir until chocolate is melted and mixture is smooth, then remove from heat and let cool.
In a large bowl, beat eggs, granulated sugar, and brown sugar with an electric mixer for 7 to 10 minutes.
Add sourdough discard and cooled chocolate mixture; beat on low speed until combined.
Fold in flour and salt with a spatula until just combined; batter will be thick.
Pour batter into prepared baking dish and smooth the top.
Bake for 35 to 40 minutes until edges are set but center remains gooey.
Cool brownies completely in the pan, then lift out using parchment paper and cut into squares.