01 - Place the cubed potatoes in a large pot and cover with cold salted water. Bring to a rolling boil over high heat, then reduce to a gentle simmer. Cook until the potatoes are fork-tender, approximately 12 to 15 minutes. Drain thoroughly and set aside.
02 - While the potatoes cook, melt 1 tablespoon of butter in a medium skillet over medium heat. Add the red bell pepper, jalapeño, green onions, and garlic. Sauté until the vegetables are softened and fragrant, about 4 to 5 minutes. Remove from heat and reserve.
03 - Return the drained potatoes to the pot. Add the remaining butter and the warm milk. Using a potato masher or hand mixer, mash until the mixture is smooth and creamy with no large lumps remaining.
04 - Gently stir in the sautéed vegetable mixture, shredded cheddar cheese, sour cream, cumin, smoked paprika, chili powder, salt, and black pepper. Continue folding until all ingredients are evenly incorporated and the cheese has fully melted into the potatoes.
05 - Taste the mashed potatoes and adjust the salt, pepper, or chili powder to your preference. For a spicier result, add a dash of cayenne pepper or leave some jalapeño seeds in the sauté.
06 - Transfer the mashed potatoes to a warm serving bowl. Garnish with chopped fresh cilantro and additional sliced green onions if desired. Serve immediately while hot.