Preheat the oven to 400 degrees F. Grease an 8-by-8-inch baking dish with butter.
Bring a large pot of salted water to a boil over high heat. Add the cavatappi and cook until just firm to the bite, 8 minutes. Drain and set aside.
Melt the butter in a large, wide pot over medium-low heat. Make a roux by sprinkling the flour over the butter and whisking to incorporate. Cook, whisking, until golden, 5 minutes. Whisk the milk into the roux until the mixture is fully combined and smooth. Still whisking, increase the heat to medium-high and bring the sauce to a low boil, 4 minutes. Reduce the heat to low and simmer until the sauce thickens enough to coat the back of a spoon, 3 minutes.
Add the mustard powder, salt and white pepper to the sauce and stir to incorporate. Add 8 ounces of the grated Cheddar and all of the Parmesan and ricotta, stirring until melted and fully blended. Remove from the heat.
Add the drained cavatappi to the cheese sauce and stir well to coat. Transfer the mixture to the prepared baking dish and scatter the remaining 2 ounces Cheddar over the top.
Bake until bubbling and the cheese on top has browned a bit, 25 to 30 minutes. Let cool for 15 minutes before serving.