For the mac, melt the butter in a large saucepan over medium heat. When the butter has melted and started to bubble, whisk in the flour. Cook for 1 1/2 minutes, whisking constantly. Gradually whisk in the milk until no lumps remain. Reduce the heat to medium-low and cook milk mixture, whisking constantly, until it thickens and bubbles, 5-7 minutes.
Remove 1/3 of sauce and save to make dipping sauce.
Remove saucepan from heat and stir in the cheese by the handful, stirring each addition until blended. Stir in the mustard, salt and pepper. Return the sauce to low heat and mix in the pasta until well-coated. Spread the mixture in a single layer on a baking sheet and let cool.
Place a spring roll wrapper on a flat work surface. Brush the edges with egg mixture.
Using a small ice cream scoop, scoop about 2 tablespoons of the mac and cheese onto the bottom third of the wrapper and in the middle of the wrapper. Fold the two edges until they meet in the middle. Roll tight and pull as you roll.
Place on a baking sheet dusted with Wondra flour to keep from sticking. (The spring rolls can be frozen at this point. Partially defrost before frying. 10-15 minutes.)
Frying: Deep fry spring rolls in three inches of oil (deep enough to cover the spring roll) at 350F for 4-5 minutes until the shell is golden brown.
Baking: Preheat oven to 350F. Place rack on a baking sheet and spray with non-stick spray. Bake spring rolls on the rack for 5-7 minutes, turning once.
For the dipping sauce, mix the reserved bechamel with sriracha to taste. Serve alongside the spring rolls.