Bacon and Pimiento Mac 'N Cheese with Spicy Breadcrumbs

Cook bacon until crisp, then transfer to a paper towel-lined plate and let cool. In a large pot of salted boiling water, cook pasta shells for 5 minutes. Drain. Meanwhile, in a large saucepan over medium heat, melt butter and add garlic. Cook until fragrant, 2 minutes. Whisk in flour, 2 minutes, then stir in milk, pimentos, Dijon, and a few splashes of hot sauce to taste and cook until almost simmering. Add cheese and stir until melted and creamy, then add drained cooked shells and stir to coat. Crumble and stir in bacon, then transfer mixture to a greased baking dish. Heat broiler and make spicy breadcrumbs: Combine toasted breadcrumbs with Parmesan and cayenne pepper. Sprinkle breadcrumbs on mac 'n cheese and broil until bubbly and golden, about 5 minutes. Garnish with chives.