1. Cook macaroni until al dente. Drain well and set aside.
2. 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.
3. Add cheese and stir until melted. Taste the sauce and add more salt as needed! Do not under-salt!
4. Add the well drained macaroni and stir to combine. Remove from flame and allow to cool slightly before assembling the egg rolls.