Preheat the oven to 350 F. Lightly coat 12 to 18 cups of a mini muffin tin with cooking spray.
Prick the sweet potato a few times with a fork and microwave until it is completely soft and tender, about seven minutes. Scoop out the flesh and set 3/4 cup in a large bowl and add the black bean puree. Whisk in the melted butter, eggs, brown sugar, and vanilla.
Add the chocolate chips to a small bowl and pour the hot coffee on top. Stir until melted, then add it to the sweet potato mixture.
In another bowl, combine the cocoa powder, baking soda and salt. Add the cocoa mixture to the sweet potato mixture and stir just until combined.
Use a spoon to fill the muffin cups three-quarters full. Bake for about 14 minutes, or until a cake tester inserted into the centre of a brownie comes out with a few moist crumbs attached and the brownies resist light pressure. Let the brownies cool in a pan for five minutes, then turn them out onto a wire rack to cool. Repeat with any remaining batter. The brownies will keep in an airtight container for up to three days.