Cook penne according to package directions until al dente. Drain and set aside to cool.
Heat oven to 375°F. Bake potatoes until tender, about 45 minutes. Let cool slightly, then peel potatoes with your hands. Peels should come off easily. Discard skins. Push potatoes through ricer into medium-sized bowl. Or, mash with potato masher. Stir in butter, truffle oil and scallions until well mixed. Season with pinch each of salt and pepper. Using fingers, press potato mixture evenly across bottom of 9-inch springform pan. Bake until lightly browned on edges, about 25 minutes. Let cool on wire rack.
Bring half-and-half to boil in medium-sized saucepan set over medium heat, stirring occasionally. Remove from heat and stir in red pepper flakes. Let mixture cool slightly, then briskly whisk in eggs, egg yolks, asiago and nutmeg. Season with more salt and pepper and set aside.
Heat vegetable oil in large skillet set over medium heat. Add shallots and cook, stirring occasionally, until softened, about 2 minutes. Add kale and cook, stirring occasionally, until wilted, about 5 minutes. Remove from heat, season with salt and pepper and set aside.
Stir cooked penne, asiago mixture, kale mixture, prosciutto, and sage into large bowl until well combined. Scrape mixture over the potato crust and spread evenly. Top with parmesan and bake until well-browned on tips of penne, about 45 minutes. Remove from oven and let cool slightly on rack before releasing springform pan. Slice into wedges and serve warm with additional parmesan passed at table.