How to Make It
Cook pasta in a large pot of boiling salted water to cover according to package directions. Drain and set aside.
Melt butter in a large skillet over medium. Whisk in flour until smooth. Cook, whisking constantly, 1 minute. Slowly whisk in milk. Cook, whisking constantly, until mixture is thickened and bubbly, 3 to 4 minutes. Remove from heat.
Whisk buttermilk, salt, pepper, and 1 cup of the cheese into flour mixture, whisking until cheese is melted. Stir in spinach, 1 cup at a time, stirring until wilted before the next addition. Add chicken and pasta; stir until coated and warmed through. Sprinkle with remaining 1/2 cup cheese.