Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper and place a wire rack on top. Spray the rack with cooking spray. Pound chicken breasts to ¾-inch thickness using a meat mallet for even cooking. For tenders, leave as is. Combine buttermilk with 1 tablespoon Dijon mustard in a large bowl. Add chicken and marinate for at least 10 minutes. Crush pretzel twists in a food processor until they form fine crumbs with some texture, similar to panko breadcrumbs. Set up three shallow bowls: first with flour, second with eggs whisked with 1 tablespoon Dijon mustard, third with crushed pretzels mixed with garlic powder, onion powder, smoked paprika, cayenne, salt, and black pepper. Remove chicken from buttermilk marinade. Dredge each piece in flour, shaking off excess. Dip into egg mixture, then press firmly into pretzel mixture, coating all sides completely. Place coated chicken on the prepared wire rack, spacing pieces 1 inch apart. Spray tops lightly with cooking spray or drizzle with olive oil. Bake for 25-30 minutes (20-25 for tenders) until internal temperature reaches 165°F and coating is golden brown and crispy. While chicken bakes, melt butter in a medium saucepan over medium heat. Add flour and whisk constantly for 1-2 minutes to create a smooth roux. Gradually pour in milk while whisking continuously. Continue whisking until mixture thickens and bubbles, about 3-4 minutes. Reduce heat to low. Add shredded cheddar cheese in three batches, stirring until each addition melts completely before adding more. Stir in both Dijon and whole grain mustards, honey, garlic powder, and cayenne if desired. Season with salt and pepper. Thin with milk if sauce is too thick. Keep warm on lowest heat. Remove chicken from oven when cooked through and golden. Let rest for 3-5 minutes. Serve crispy pretzel chicken immediately with warm mustard-cheddar sauce on the side or drizzled over top. Garnish with fresh parsley if desired.