Cook the pasta according to package directions until just al dente. Drain and reserve. While the pasta cooks, melt the butter in a medium saucepan over medium heat. Stir in the flour and cook, stirring, for 1 minute. Slowly add the milk, whisking to prevent lumps from forming. Simmer the béchamel for 5 minutes, until it begins to thicken to the consistency of heavy cream. Stir in the Cheddar and 1⁄4 cup of the Parmesan, and cook until completely melted. Cut the heat and stir in the yogurt. Add the pasta and toss to evenly coat. Preheat the broiler. Pour the macaroni and cheese into an 8" x 8" baking dish (or into individual ramekins). Top with the bread crumbs and the remaining Parmesan and season with black pepper. Place on the middle rack of the oven and broil for 5 to 7 minutes, until the bread crumbs are golden brown.