Preheat the oven to 350ºF and grease a mini muffin tin with coconut oil to help prevent sticking.
In a food processor fitted with an "S" blade, combine the dates and water and process until the dates have broken down into a paste. (They may turn into a ball in the processor, which is also okay.)
Add in the cashew butter, ground flax, cacao powder, baking soda, vanilla, and vinegar and process again until a sticky, uniform dough is created. (If you need to add 1 more tablespoon of water to help it blend smoothly, that's okay.) The batter will be thick and sticky.
Use a cookie scooper to help you scoop the dough into the greased mini muffin tin. (Roughly using 1 tablespoon of dough per slot.) Use your fingers to gently flatten each mound of dough. Because the dough is very sticky, it helps to get your fingers wet with water before pressing each mound down so your fingers won't stick to the dough.
Bake until the tops look dry, about 12 to 15 minutes. (The centers will still feel slightly soft to the touch.) Remove from the oven and let the brownies cool completely, at least 30 minutes, before attempting to remove them from the pan. They firm up as they cool, but are very fragile when warm. Run a knife around the edge of each brownie to help loosen them to be removed, then serve immediately or store them in the fridge in an airtight container for up to 2 weeks.
If you don't mind a bit of added sugar, you can add up to 1/2 cup of mini chocolate chips into this recipe for a double chocolate brownie.