Preheat oven to 350˚F/175˚C. Grease an 8-inch square pan with vegan butter or spray then line the pan with parchment paper to come up all sides of the pan with a bit of overhang. Then grease the top of the parchment paper as well.
Melt the butter in a small pan on the stovetop or in the microwave. Meanwhile, chop the dark chocolate into small pieces using a chocolate bar. Once the butter is melted stir in the chocolate pieces off heat until the chocolate is melted and the mixture is smooth for about 30 seconds.
In a large mixing bowl, whisk together vigorously the aquafaba, sugars, brown sugar, melted butter/chocolate mixture, vanilla extract, and espresso powder for a full two minutes. This step may also be done with a hand mixer or in a stand mixer.
Place a fine-mesh strainer over the bowl and sift the flour, cocoa powders, salt, and baking powder into the wet mixture. Stir with a rubber spatula until evenly incorporated into a thick sticky batter. If adding any mix-ins such as mini chocolate chips or nuts to the batter fold them in at this point. Add only half of the nuts and reserve the other half to sprinkle on top.
Spread the batter into the prepared pan and smooth it out with a rubber spatula.
Bake the brownies for 20-25 minutes until the edges puff up slightly and the top is shiny and a toothpick comes out with moist crumbs.
Cool the brownies in the pan on a cooling rack. Once they are cool to room temperature lift them out of the pan using the parchment paper overhangs. Slice them into 9 squares, wiping the knife off with a damp paper towel between each cut for clean cuts.
To make dark chocolate espresso ganache heat coconut milk, vanilla, and instant espresso in a small pan just before the mixture boils. Do not let it boil. Remove from heat and whisk in dark chocolate chips or pieces. Keep whisking until the chocolate has melted and the mixture is silky smooth. Let ganache cool at room temperature for about 15 minutes. Use the ganache in 1 of 2 different methods: either pour and spread ganache over uncut brownies in the pan and allow it to set before cutting or drizzle ganache over uncut or cut brownies (as shown in pictures).