Preheat your oven to 350°F (175°C) and prepare a 7x11 inch baking pan by greasing it lightly with butter or lining it with parchment paper.

In a medium saucepan over low heat, melt the unsalted butter. Once melted, whisk in granulated sugar and brown sugar until smooth. Add eggs one at a time, mix thoroughly, then stir in vanilla extract and sourdough starter discard.

In a separate bowl, sift together all-purpose flour, unsweetened cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, folding gently until combined. Fold in dark chocolate chips or chunks.

Pour the batter into the prepared pan, spreading evenly. Bake for 25-30 minutes, checking for doneness with a toothpick.

Allow the brownies to cool in the pan for 10-15 minutes before transferring to a wire rack and cutting into squares.