Cook pasta according to package directions until al dente, drain well, and set aside.
In a large pot over medium heat, melt butter and whisk in flour, cooking for 2 minutes until it forms a paste.
Slowly pour in milk while whisking constantly until mixture is smooth and begins to thicken, about 5-7 minutes.
Reduce heat to low and stir in white cheddar and fontina cheeses until completely melted and smooth.
Whisk in Dijon mustard, salt, black pepper, and cayenne pepper until well incorporated.
Add cooked pasta to the cheese sauce and stir gently until every piece is coated, then serve hot.