Bake potatoes at 400° on oven rack 1 hour or until soft. Cool; remove skins.
Place potatoes in a large bowl; add butter. Beat potatoes at medium speed with an electric mixer until mashed. Stir in jalapeños, 1 cup cheese, salt, and pepper.
Spoon mixture into a lightly greased 9- x 13-inch or 3-quart shallow baking dish; sprinkle remaining 1 cup cheese over top. Bake at 400° for 30 minutes or until the edges are bubbly and the cheese is melted.
Heat broiler. Broil casserole 3 minutes or until cheese is golden and crispy.