1. Puree 1 cup buttermilk with onion and garlic in a blender or food processor until smooth and combined. Transfer mixture to a large bowl; whisk in remaining 3 cups buttermilk, hot sauce, and 2 tablespoons salt. Add chicken and let soak in refrigerator 4 hours, keeping chicken fully submerged (with a plate, if necessary).

2. Whisk together flour, cayenne, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a shallow dish. Working with a few pieces at a time, remove chicken from buttermilk mixture, letting excess drip back into bowl. Dredge in flour mixture to coat. Transfer chicken to a rack set over a rimmed baking sheet and let stand 30 minutes.

3. Heat a large, heavy-bottomed pot containing 2 inches oil over medium-high until a thermometer registers 350 degrees. Working in batches, carefully add chicken to pot; cook, turning occasionally with tongs, until golden brown and a thermometer inserted into thickest parts (avoiding bone) registers 165 degrees, about 15 minutes for white meat, 10 to 12 minutes for dark. (Adjust heat to maintain oil temperature between 325 degrees and 350 degrees, and return oil to 350 degrees between batches.) Transfer to a paper towel-lined plate and season with salt and pepper. Serve warm, drizzled with honey and sprinkled with lavender.