Buttermilk marinade is the secret to fried chicken that’s tender and juicy. I’ve suggested boneless breasts here, but don’t hesitate to use any boneless or bone-in chicken parts. At my house, fried chicken always gets served with mashed potatoes and gravy. The enzymes in buttermilk act as a natural tenderizer and the jalapeño provides a nice fresh kick of flavor in this marinade. It’s a perfect start for fried chicken, but also works well for grilled foods. — Ray "Dr. BBQ" Lampe, author of Flavorize: Great Marinades, Injections, Brines, Rubs, and Glazes.
Heat the olive oil in a medium skillet over medium-low heat. Add the onion, jalapeños, and garlic and cook them, stirring often until they are soft, 7 to 8 minutes. Transfer the vegetables to a plate to cool.
In the pitcher of a blender, combine the buttermilk, salt, pepper, and jalapeño mixture. Blend them on high speed for 30 seconds, until they are well combined. This may be made up to 12 hours ahead and kept covered in the refrigerator.
Place the chicken breasts on a cutting board and pound them gently with a meat mallet just until they are an even thickness. Place the breasts in a large heavy-duty zip-top bag. Pour the marinade over them. Seal the bag, squeezing out as much air as possible. Move the breasts around within the bag so they are well-coated with the marinade. Refrigerate them for 4 to 6 hours, occasionally moving the breasts around in the bag to make sure they remain well coated.
On a large flat plate, combine the flour, breadcrumbs, cornstarch, salt, pepper, granulated garlic, and granulated onion. Remove the breasts one at a time from the marinade, shaking off any excess. Dredge the breasts in the flour mixture, coating them well on all sides. Lay each breast on a wire rack set over a baking sheet to rest. Let them rest for 15 minutes.
Add enough vegetable oil to a large skillet to reach a depth of ½-inch. Heat the oil over medium-high heat to a temperature of 325 degrees F. Two at a time, add the chicken to the hot oil. Cook the chicken until it is golden brown on the bottom, 6 to 7 minutes. Flip the breasts over and cook them until they’re golden brown and reach an internal temperature of 160 degrees F, another 6 to 7 minutes. Transfer them to a clean wire rack to rest for 5 minutes before serving. Serve the chicken family-style, allowing for one whole breast per person.