Preheat the oven to 350°F. Line an 8×8 baking dish with parchment paper, leaving some overhang for easy removal later.

In a stand mixer fitted with the whisk attachment (or using a hand mixer), combine the aquafaba and granulated sugar.

Whip on medium speed for 2–3 minutes, until the mixture looks lighter in color and slightly fluffy. This step is essential for the brownies to bake correctly.

In a microwave-safe bowl, add the chopped plant-based butter and 1 cup of the chocolate chips.

Microwave in 30-second intervals, stirring between each, until fully melted and smooth. Set aside to cool slightly.

To the whipped aquafaba mixture, add the vanilla extract, espresso powder, and salt. Mix briefly until combined.

Slowly pour the melted butter and chocolate mixture into the aquafaba mixture. Mix on low speed just until incorporated.

Sift together the flour and cocoa powder.

Add them to the wet mixture and gently fold using a spatula until just combined and no dry streaks remain.

Do not overmix - stop as soon as everything comes together.

Gently fold in the remaining ½ cup of chocolate chips or chopped chocolate.

Pour the batter into the prepared pan and smooth the top.

Bake for 35 minutes, checking near the end to avoid overbaking. The center should be set but still soft.