Spoon the batter (it will be quite thick) into the muffin tin. Smooth it with a spatula and sprinkle chopped pistachios on top. Bake on middle shelf for about 11-12 minutes. Remember the brownies will continue to cook for a little while longer once removed from the oven and the tops will harden as they cool. The top should be nice and crispy, but the inside gooey. Remove them from the oven and let them sit in the tray until they are cool enough to remove. If you try to remove them too quickly, they may fall apart.
*Aquafaba (AF) or bean brine is leftover cooking water from cooking most legumes. I use chickpea water and have not tried using any other type yet. You can get it from a can of low sodium chickpeas or make your own. In this recipe, I used homemade aquafaba. When making your aquafaba, soak dry chickpeas in lots of water overnight, rinse and put in a large pot with lots of water. Bring to the boil and then simmer for about 1 hour (until beans are soft). Remove cooked chickpeas with a slotted spoon and what’s left in your pot is aquafaba. If it is too runny (ideally it should resemble egg whites), reduce it a bit by simmering it on a low heat (without a lid) until it reaches the consistency similar to an egg white. Cool it down before using.