Cook potatoes in boiling salted water to cover 12 minutes or until tender. Drain potatoes, and transfer to a large bowl. Mash with a potato masher until smooth; stir in cream, butter, salt, and pepper. Let stand until cool enough to handle.
Shape about 1/4 cup potato mixture into 1 1/2- to 2-inch patties. Brush patties with egg, and dredge in panko. Heat oil in a large skillet over medium heat. Add potato cakes; cook, in batches, 2 to 3 minutes on each side or until golden brown on both sides. Drain on paper towels, and top with Smoked Trout, Apple, and Horseradish Cream.