Fill a large pot with salted water and bring to a boil. Pour the olive oil into the skillet, and set over medium-high heat. Drop in the butter, let it melt, and then scatter in the bacon pieces. Cook the bacon, stirring and tossing the pieces, for 5 minutes or so, until they’re crisp and caramelized. (If the bacon rendered a lot of fat, you can pour off half the fat from the pan.) Crumble the bacon into small pieces. Add in the kale and cook until wilted – stirring occasionally. Beat the eggs and ¼ teaspoon of salt in a bowl until well blended. When the water is at a rolling boil, cook cavatelli according to packaging instructions, lift them from the pot, drain briefly and add them into the skillet. Over medium heat toss the cavatelli with the bacon pieces, coating the pasta with the olive oil, butter and bacon fat. Pour the beaten eggs all over the pasta, stirring and tumbling them together; keep scraping the coagulating egg from the sides and bottom of the pan and incorporate it in with the wet eggs. Cook for 1-2 minutes, just until all the egg is lightly cooked and custardy and evenly scrambled into the cavatelli. Turn off the heat. Scatter the shredded fontina cheese over the eggs and cavatelli, toss thoroughly to blend in the cheese as it melts. Serve immediately.