Preheat oven to 375 degrees. In a small pan or skillet, melt 1 T of butter and toss with breadcrumbs; toast on sheet pan in oven until lightly browned, approx. 5-7 min; set aside. Boil pasta in lightly salted water until al dente, approx. 8 min, stirring occasionally. In a lg pot over med heat, melt 3 T butter. Add flour and whisk 3 min until a fragrant roux forms. Quickly add salt, pepper, mustard, paprika, milk and hot sauce. Bring to a simmer, stirring constantly, until sauce thickens slightly, approx. 6 min. Slowly add cheeses, reserving 2 c cheddar. Stir until smooth and completely melted. Remove from heat, taste and adjust seasonings if necessary. Add drained pasta, 1 T chives and 3/4 of bacon; reserve remaining bacon and chives for garnish. Pour 1/2 of pasta into a 2-quart casserole dish. Top with 1 1/2 c of shredded cheddar, followed by remaining pasta; do not stir (this creates a melted cheesy layer in the middle of the dish). Top pasta with 1/2 c cheddar, breadcrumbs and remaining bacon. Dust lightly with smoked paprika. Bake until bubbly and lightly browned on top, approx. 30 min. Garnish with 1 T chopped chives and serve immediately.