Cook bacon in a large skillet over low heat until crispy. Transfer to paper towels to drain. Drain all but 1 tablespoon bacon grease from the skillet.
Place sweet potatoes in a large pot of water with 1/2 teaspoon kosher salt. Bring to a boil, reduce heat to low, cover and simmer until potatoes are very tender, about 15 minutes.
While potatoes are cooking, add garlic and kale to remaining bacon grease in skillet. Saute on medium-low heat, working in batches if needed, until kale is crisp-tender and reduced to about half of its volume. This will take 5 to 10 minutes. Remove skillet from heat.
Drain sweet potatoes and return to them to their pan. Add butter or vegan butter and mash with a potato masher or fork. Crumble the bacon and stir into potatoes. Stir in sauteed kale and season with remaining 1/2 teaspoon kosher salt and 1/4 teaspoon pepper (or to taste). Serve.