For two people:
Cook pasta according to pack.
Melt butter in a small pan. Add flour and cook on gentle heat for 1-2mins, stirring continuously.
Remove pan from heat and add all the milk at once. Stir well then return to heat and cook gently stirring constantly for about 5min. The sauce will thicken. If too thick, thin out with milk, after which you might need to reheat it.
Add 3/4 of the cheese to the sauce and stir until melted in. Combine the sauce with the pasta in a heat-proof bowl. Top with the remaining cheese and (optionally) with breadcrumbs.
Place under hot grill and grill until cheese is golden and bubbly.