Preheat your oven to 325°F. While the oven heats, line an 8x8 inch baking dish with parchment paper, allowing some overhang on the sides for easy removal later. This setup means your brownies will be ready to go into the oven as soon as the batter is mixed.

In a medium bowl, whisk together the flour, sifted cocoa powder, salt, and instant espresso powder. The espresso powder won't make the brownies taste like coffee—instead, it deepens and intensifies the chocolate flavor. Set this dry mixture aside while you work on the wet ingredients.

In a microwave-safe bowl, combine the vegan butter and 1/2 cup melted vegan chocolate chips. Microwave in 30-second intervals, stirring between each, until completely smooth and melted—this takes about 1-2 minutes total. Let the mixture cool slightly, then stir in the granulated sugar and vanilla extract until well combined. This cooling step is important because it prevents the aquafaba from breaking down when you add it next.

Pour the aquafaba into a bowl and whisk it vigorously for 2-3 minutes until it becomes foamy with visible peaks—this aerates the batter and helps create lift and a fudgy texture in the brownies. Gently fold the whipped aquafaba into the cooled chocolate mixture from Step 3 until just combined. I find that using a sourdough starter in vegan baking really enhances the brownie's depth, so next stir in the active sourdough starter until fully incorporated.

Add the dry ingredient mixture from Step 2 to the wet batter from Step 4, folding gently with a spatula until just combined—don't overmix, as this keeps the brownies tender. Fold in the reserved 1/4 cup vegan chocolate chips, distributing them evenly throughout the batter.

Pour the batter into your prepared pan from Step 1, spreading it evenly. Bake at 325°F for 30-35 minutes—the brownies are done when a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). Let the brownies cool in the pan for 15 minutes, then lift out using the parchment paper overhang and transfer to a wire rack to cool completely before slicing. I find that this cooling time in the pan helps them set properly without becoming too dry.