In a separate, heavy-bottomed saucepan, melt the butter over medium-high heat. Whisk in the flour and cook for 1 to 2 minutes, whisking constantly, until it forms a smooth, bubbly paste. (Be careful not to let it brown.) Pour in the hot milk and whisk for 3 to 5 minutes or until the sauce thickens. Season to taste with salt and pepper, then bring the sauce to a boil, reduce the heat to low, and cook for 2 more minutes. Stir in the paprika, mustard, ¾ cup of the Cheddar, and ¾ cup of the fontina. Stir until the cheeses are completely melted, then add the chives, heavy cream, and pasta, scraping any extra oil from the bottom of the pot. Mix gently until well combined, then taste and season with additional salt, pepper, and truffle oil as desired.