Melt butter over medium heat in a small saucepan. Whisk in flour until well-combined, then whisk in milk, salt, pepper, cayenne pepper and 1 cup of Monterrey Jack cheese. Reduce heat to simmer and continue to whisk until sauce simmers slightly around the edges and has thickened slightly. Stir in sour cream.
Add broccoli florets to a large heavy-bottomed stockpot of boiling water. Cook for 3-5 minutes and then drain broccoli from the water. Set aside.
Preheat oven to 350º F. Spray a 13 x 9 - inch baking dish with nonstick cooking spray.