Place butter in a medium skillet. Melt over medium-high heat, then quickly whisk in flour until smooth.
Very slowly, whisk milk and heavy cream into flour mixture until smooth. Bring sauce to a low simmer, stirring occasionally until sauce is slightly thickened.
Remove from heat and whisk in Parmesan and pesto until smooth.
Meanwhile, prepare pasta according to recipe instructions, reserving 1 cup pasta cooking water.
Pour sauce over pasta and gently toss until noodles are coated.
Slowly drizzle in reserved pasta water until pasta is creamy.
Season pasta with salt and pepper and serve warm with additional cheese if desired.