Cook the pasta in a medium saucepan, according to the package instructions. You want it al dente.
Melt the butter in a large saucepan on medium heat.
Add in the flour and whisk for 2-3 minutes.
Pour in the milk a little at a time, as you whisk, to prevent clumps.
Once the sauce looks a thick dip, pour in the remaining milk.
Turn up the heat to medium-high and let the sauce thicken for a few minutes while you stir.
Once the sauce is thick enough to stick to the back of a spoon, season to taste with sea salt and pepper.
Turn down the heat to medium.
Add the cheese and stir to combine.
When the pasta is cooked, pour it into the sauce.
Use a wooden spoon to combine.
Serve.