In a saucepan over low heat, melt the butter and chocolate together, stirring until smooth. Remove from heat and cool slightly.

In a bowl, vigorously whisk the sugar and eggs until pale and thickened, about 1 minute. Whisk in the vanilla and the melted chocolate mixture, then stir in the sourdough discard just until smooth.

Sift the flour, cocoa powder, and salt into the wet mixture. Fold gently with a spatula until no dry streaks remain. Fold in the chocolate chips or walnuts if using.

Preheat the oven to 350°F (175°C). Scrape the batter into an 8×8-inch parchment-lined pan and smooth the top. Bake for 25–30 minutes, until the edges are set and the center jiggles slightly.

Cool in the pan for at least 20 minutes, then lift out, slice into squares, and serve.