Bring a pot of salted water to a boil. Add the elbow macaroni and cook according to the package directions. Drain and set aside.
Meanwhile, place the chicken skin in a large nonstick saute pan over medium heat, being careful to keep the skin as flat as possible. Cook, flipping over halfway, for about 2 minutes, until the skin is crisp and the fat has rendered out. Remove the skin from the pan and place on a paper towel-lined plate.
Melt 1 tablespoon of the butter in the same pan over medium-high heat. Add the spinach and wilt for 1 minute, stirring occasionally. Remove the spinach from the pan and set aside.
To the same pan, add the remaining 3 tablespoons of butter and melt over medium-high heat. Add the flour and cook, whisking continuously, until the mixture is free of any lumps and light tan in color, about 1 minute. Lower the heat to medium, slowly pour in the milk, whisking constantly, and cook until the mixture is slightly thickened, about 2 minutes. Add the cheese, whisking to combine, and cook until it is melted and the sauce is smooth. Reduce the heat to low and whisk in the chicken stock, lemon juice, yellow mustard and hot sauce. Season the sauce with salt and pepper.
Add the drained pasta, wilted spinach and pulled chicken to the sauce. Stir to combine. Crumble the crispy chicken skin on top and serve immediately.