01 - Bring a large pot of salted water to a rolling boil. Add pasta and cook according to package directions until al dente. Reserve 1/2 cup of starchy pasta water before draining. Set drained pasta aside.
02 - In a large skillet set over medium heat, melt the butter until it begins to foam. Add the minced garlic and sauté for 1 minute, stirring constantly, until fragrant and lightly golden.
03 - Stir in the Dijon mustard, smoked paprika, crushed red pepper flakes, and Worcestershire sauce. Pour in the lemon zest and juice. Cook for 1 to 2 minutes, stirring frequently, allowing the flavors to meld together.
04 - Add the chopped parsley, chives, kosher salt, and black pepper. Stir well to combine all the herbs and seasonings evenly throughout the buttery mixture.
05 - Transfer the cooked pasta into the skillet with the cowboy butter sauce. Toss vigorously, adding splashes of the reserved pasta water as needed until a glossy, emulsified coating clings to every strand.
06 - If using shredded chicken or grilled steak, fold it into the pasta now. Continue cooking for 2 minutes until the protein is heated through and evenly distributed.
07 - Divide among plates immediately. Finish with extra fresh herbs, a generous shower of grated parmesan, and lemon wedges alongside for squeezing.