Preheat oven to 400 degrees. Mix butter and mashed potato. Sift in flour add salt and mix by hand. Knead, wrap and chill for 30 minutes. Roll out dough to 1/3-inch thick. Cut into 3-inch strips, brush with egg, top with fennel and bake on a greased sheet pan for 10 to 12 minutes. Sprinkle with salt and pepper.