You can do this while the pasta is cooking.
In a large saucepan, melt the butter over low heat.
When the butter is completely melted whisk in the flour, white pepper, mustard powder, and kosher salt to taste.
When the flour is fully incorporated, pour in the milk. Turn your burner to medium, stir often, until the mixture comes to a boil. Let the mixture boil for 1 minute, stirring constantly.
Add the pasta to the cheese sauce.
Divide the mac & cheese between your ramekins. Place the ramekins on a sheet pan just in case anything drips out and to make it easier to remove the ramekins from the oven.
Bake for 25-30 minutes or until the mac & cheese is brown and bubbly on top. Let cool for at least 5 minutes before enjoying.