First, make the sweet potato purée. Peel and slice the sweet potato into 2-inch chunks, and bring a large pot of water to a boil. Add the potatoes and boil until tender, about 15 minutes, then drain and transfer to a mixing bowl.
Roughly mash the sweet potato and then add butter and milk. Whip with a hand mixer on low speed at first, increasing to medium-low speed as the ingredients are incorporated.* Once the sweet potato purée is smooth and creamy, season with salt and set aside.
Next, make the taco filling. Cook the chorizo in a frying pan over medium-high heat until browned, about 8 to 10 minutes. Reduce heat to medium, then add the black beans and cook until warmed through, about 1 minute.
Finally, assemble the tacos. Warm corn tortillas on a cast iron skillet over medium heat until pliable, then fill each tortilla with about 1 tablespoon of the sweet potato purée before adding the chorizo and black bean filling.
Top tacos with raw onion and fresh cilantro, then finish with a lime squeeze. Serve immediately.