Cut the chicken breasts lengthwise, like a butterfly fillet, but cut all the way through. Pound to an even thickness, if necessary. These thinner cuts cook faster and dry out less.
Combine all of the herb mix ingredients in a small bowl with a lid. Close and shake to mix.
Measure one cup of buttermilk into a measuring cup, then add 1 tbsp (15 mL) of herb mix. Stir. Place chicken in a glass baking dish and pour buttermilk mixture over. Cover with a lid or plastic wrap and refrigerate for 4-6 hours or overnight. Set aside the herbs for use in the breading mix.
Remove chicken from fridge about an hour before cooking time. While it comes to room temperature, prepare the breading mix. After processing the ranch croutons into coarse crumbs, pour them into a 1-cup (240 mL) measuring cup. It will not be full. Add panko bread crumbs to make one cup. (Or just go all panko, as described in the notes below.) Add the bread crumbs to a large bowl, then add the flour, cornmeal, and remaining herb mix. Stir. (You can also add all of these ingredients to a large freezer bag if you prefer to “shake and bake.”)
About 15 minutes before cooking, preheat oven to 400F (205C). Line a rimmed baking sheet with parchment–not foil because food sticks to it. Cut up the butter and lay it around the pan. Put the pan in the oven to melt the butter while the oven heats. Remove it immediately when the butter is melted.
Remove chicken from marinade and dab off excess liquid with paper towels. Add chicken, one piece at a time, to the bowl of breading mix. Press breading into the chicken to ensure it is thoroughly coated. Place breaded chicken on a plate. Repeat with the remaining pieces. (If using the shake & bake method, put two chicken pieces at a time into the bag with the breading mix, then shake to coat.)
When all the chicken is breaded, place on the baking pan in the melted butter. Bake for about 6-8 minutes, until edges start to turn golden, then carefully turn the chicken. Bake for another 4 minutes or so, until the chicken registers 165F (74C) on a thermometer. Broil for a few minutes to make the coating crispy.
Remove from the oven. Serve chicken with a side of thick ranch dressing for dipping or to drizzle over the chicken. (See Notes, below.)