Heat oven to 425°F. Line rimmed baking sheet with nonstick foil. Cut chicken in half horizontally to make thin pieces, then cut each piece in half crosswise.  Place cornflakes on plate. In medium bowl, combine 1/4 cup buttermilk, garlic powder, 1/4 teaspoon cayenne and 1/2 teaspoon salt. Add chicken and toss to coat. Working with 1 piece at a time, remove chicken, letting any excess buttermilk drip off, then coat in cornflakes, pressing gently to help them adhere. Transfer to prepared baking sheet. Roast, rotating pan once, until chicken is cooked through, 10 to 12 minutes. Meanwhile, make slaw: In large bowl, whisk together sour cream, vinegar, poppy seeds, remaining 1/4 cup buttermilk, pinch cayenne and 1/4 teaspoon salt. Add cabbage and toss to coat. Fold in scallion. Form sandwiches with focaccia, chicken, slaw and hot honey.