Preheat oven to 425 degrees F (220 degrees C). Butter a 9×13-inch baking dish with 1 tablespoon butter.
Melt butter in a saucepan over medium heat. Whisk flour into butter; cook and stir until raw flour taste cooks off, 2 to 3 minutes.
Gradually whisk milk into flour mixture. Increase heat to medium-high; cook and stir until mixture thickens and comes to a simmer, 5 to 10 minutes. Stir truffled pecorino cheese, salt, cayenne pepper, and nutmeg into milk mixture until cheese sauce is thick.
Bring a large pot of salted water to a boil; add cauliflower and cook until slightly tender, about 6 minutes. Drain and transfer cauliflower to prepared baking dish. Spread cheese sauce evenly over cauliflower. Dust with bread crumbs, Parmigiano-Reggiano cheese; drizzle olive oil over the top and sprinkle with a pinch of cayenne pepper.
Bake in the preheated oven until browned and bubbling, about 30 minutes. Garnish with chopped fresh chives.