If using homemade sweet potato puree, see how to make it above. To make the brownies, start by preheating the oven to 325°F. Line an 8-inch pan with parchment, or grease well. If not easily stir-able, gently heat nut butter to soften. Whisk it with the sweet potato and vanilla until smooth. Stir all remaining ingredients together (making sure the salt and baking soda are evenly incorporated), then pour dry into wet. Stir to form a batter. Smooth into the baking pan, scooping out all traces of batter. Use a second sheet of parchment to really smooth everything down. Bake 20 minutes. They should look underdone when you take them out. The brownies firm up as they cool. If for some reason yours are too gooey after cooling, loosely cover and refrigerate a few hours. Frost if desired.