Cook macaroni until al dente. Drain well and set aside.

In a six-quart pot, melt butter and sprinkle in flour. Whisk together over medium heat for several minutes, whisking constantly. Taking care not to let it burn. Pour in milk, add the mustard, black pepper, and Kosher salt, and whisk until smooth. Cook until very thick, about five minutes. Reduce to low heat.

Add cheese and stir until melted. Taste the sauce and add more salt as needed! Do not under-salt!

Add the well drained macaroni and stir to combine. Remove from flame and allow to cool slightly before assembling the egg rolls.

To roll egg rolls, place egg roll wrapper with one corner facing you. Place approximately 3/4 cup of mac and cheese mixture on the wrapper. Fold corner over mixture, and then fold outside corners over the mixture, making a roll, four to five inches wide. Roll tightly taking care not to tear the wrapper, seal the final edge with water. Repeat with remaining wrappers.

In a pot or a deep frying pan, heat enough oil to cover egg rolls. Deep fry until golden brown, about three to four minutes. Drain on sheet pan on which a cool rack has been placed.

Serve warm, with marinara sauce on the side.