Combine grated potatoes, onion, salt, chili powder, and pepper in a large mixing bowl; toss lightly.
Heat 2 tablespoons butter and 2 tablespoons bacon drippings in a heavy 10-inch skillet over medium heat. Add potato mixture, spreading evenly over bottom of skillet using a spatula. Press potatoes down forming a flat cake. Use spatula to loosen edges from side of skillet, if necessary. Cook 10 minutes or until bottom is browned and crusty.
Remove skillet from heat. Place a large plate over skillet; invert potato cake onto plate.
Melt remaining butter and bacon drippings in skillet over medium heat; slide potato cake back into skillet. Cook 10 minutes or until bottom is browned and crusty. Invert onto a serving plate. Cut into wedges, and serve.