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Preheat the oven to 350 degrees F and line the baking pan with parchment paper. Make sure to leave enough overhang to pull the brownies out. Tip: wet the parchment paper to help with molding it into the pan.
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In a small saucepan melt the butter and stir in the cocoa powder over low to medium heat, stirring occasionally. Set aside to cool for a few minutes.
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While the mixture is cooling, beat the sugar, brown sugar, and eggs together for 2-3 minutes until light and fluffy. Make sure the sugar is fully dissolved (this helps create those beautiful crinkle tops!).
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Then add the butter and cocoa mixture along with the vanilla extract, whisking until combined. Finally, mix in the sourdough discard until fully incorporated.
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In a separate bowl, mix the flour, baking powder, salt, and espresso powder (if using).
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Gradually add the dry ingredients to the wet mixture and stir until mostly combined. Fold in the chocolate chips and mix just until no flour streaks remain...be careful not to overmix or your brownies will turn out cakey instead of fudgy.
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Lastly, spread the batter evenly into the prepared baking pan and smooth it out with a spatula or icing tool.
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Bake the sourdough brownies for 35-40 minutes (check them around 30 minutes) They're done when a toothpick inserted in the center comes out with just a few moist crumbs...not completely clean.
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Once complete, allow the brownies to cool in the pan completely (30-40 minutes) before slicing them into pieces and enjoying!
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Storage: Store brownies in an airtight container at room temperature for up to 3 days, or refrigerate for up to 1 week. For longer storage, freeze in a freezer-safe container for up to 3 months.