Combine chicken with 1/2 cup buttermilk and 1/4 teaspoon salt in a bowl and refrigerate for 15 minutes. Mix flour, cornstarch, paprika, coating garlic powder, pepper and 1/8 teaspoon salt in a shallow dish.
Mix yogurt, the separate 2 tablespoons buttermilk, mayonnaise, dill, chives, ranch garlic powder, onion powder and remaining 1/8 teaspoon salt. Refrigerate this ranch until assembly.
Cook bacon in a large skillet over medium heat for 8 to 10 minutes until crisp and transfer to a rack. Pour off the fat and wipe the skillet clean.
Heat frying oil in the skillet to 350°F. Lift chicken from its marinade, let excess drip off and press each piece firmly into the flour mixture; discard the raw marinade.
Fry chicken in two batches for 4 to 5 minutes per side, keeping oil around 350°F, until golden and 165°F in the thickest parts. Drain on a clean rack.
Warm split croissants in a 300°F oven for 5 minutes. Spread ranch over their cut faces and divide cabbage among the bottoms.
Add one fried thigh and one bacon strip to each bottom, breaking bacon to fit if needed. Close gently and serve while the chicken coating is crisp.