Sprinkle half of the cheddar onto an extra-large nonstick skillet in four 4-inch mounds. Cook over moderate heat until the cheese is lacy and golden, about 4 minutes. Using a flexible spatula, carefully flip each crisp and cook until golden, about 1 minute longer. Drain on a paper towel–lined plate and break into large pieces.
In a very large saucepan, melt the butter. Whisk in the flour and paprika and cook over moderate heat until bubbling, about 2 minutes. Add the milk and whisk until smooth. Cook over moderate heat, whisking constantly, until thick and creamy, about 5 minutes. Stir in the remaining cheddar and the Fontina and cook over low heat until melted. Off the heat, stir in the mascarpone and season with salt.
Meanwhile, cook the macaroni in a large pot of salted boiling water until al dente. Drain and add to the cheese sauce. Cook over low heat, stirring constantly, until hot and bubbling, about 2 minutes. Spoon the mac and cheese into bowls, top with the cheese crisps and serve.