Peel sweet potatoes. If they are large, cut them in half crosswise to make them more manageable. Trim a piece from the long side of the sweet potato to create a flat surface. Place the flat surface on the cutting board for stability and cut the potato into planks about 1″ in width. Then cut the planks into the same size diced potatoes.
Place the potatoes into a medium saucepan or pot and cover with cold water. Bring to a simmer over medium heat. Turn down if needed once the potatoes are cooking. You don’t want to boil them(they will fall apart). Cook until the potatoes are very soft when pierced with a sharp knife tip.
Drain the potatoes and place them back in the pan on the warm (but turned off) burner for a few minutes. This will steam off excess moisture. Mash the potatoes with an electric handheld mixer or an immersion blender with the whip attachment. Add butter or olive oil and rosemary. Serve warm.