Preheat oven to 375º F and lightly grease a baking dish with nonstick spray.
Melt butter in a medium pan over medium heat and sauté panko breadcrumbs, stirring continuously to avoid burning, until golden brown. 4-5 minutes.
Remove from heat and place panko in a shallow bowl. Whisk in taco seasoning until incorporated.
In a large bowl, combine cream cheese, mexican cheese blend and minced jalapeño.
Using a sharp knife, cut chicken breasts horizontally until almost butterflied (creating a pocket in the center), then scoop 1/4 cup cream cheese jalapeño mixture into the center of each chicken breast.
If necessary, use a toothpick to keep filling in, then season both sides of chicken generously with salt and pepper.
Beat egg in a shallow bowl or dish and set next to the breadcrumbs. Dip each stuffed chicken breast in beaten egg, shake off excess, then press into taco-seasoned panko, pressing firmly so it adheres.
Transfer coated chicken breast to greased baking sheet and continue with remaining chicken.
Place baking sheet in oven and bake for 30 minutes, or until chicken is cooked through. Recipe adapted from Key Ingredient