Bring a large stockpot of salted water to a rolling boil. Add pasta and cook according to package instructions until al dente, about 8-10 minutes. Drain and reserve some pasta water.

Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for about 1 minute.

Gradually pour in milk while whisking continuously. Cook until the sauce thickens and coats the back of a spoon, about 3-5 minutes.

Remove from heat and season with salt. Stir in pesto until fully incorporated.

Return drained pasta to the pot and pour over the creamy pesto sauce. Toss gently until fully coated, adding reserved pasta water if needed.

Serve hot, garnishing with extra pesto or grated cheese if desired.