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.
Notes
Making the sweet potato purée: The amount of butter and milk you'll need to add to the potatoes will vary based on the size of your potato. Use the amounts listed above as a starting point, but don't be afraid to add more if needed to achieve the right texture. The purée should be smooth and creamy, with no chunks in it!Finishing the tacos: Raw onion, fresh cilantro, and a squeeze of lime juice adds so much freshness and zesty flavor to these tacos - it really helps brighten up the heavy, spicy chorizo and creamy sweet potato. Don't skip these toppings!