Place chicken thighs in a large bowl or zip-top bag. Pour buttermilk over chicken, seal, and refrigerate for at least 4 hours or overnight.
In a shallow dish, whisk together flour, cornstarch, salt, and pepper.
Remove chicken from buttermilk, letting excess drip off. Dredge each thigh in flour mixture, pressing firmly to coat. For extra crunch, dip back into buttermilk and dredge again.
Heat about 1 inch of oil in a heavy skillet to 350°F. Fry chicken in batches, 4-5 minutes per side, until golden and internal temperature reaches 165°F. Transfer to a wire rack and keep warm.
In a wide shallow bowl, whisk together eggs, heavy cream, 1 tablespoon maple syrup, a pinch of salt, and vanilla extract if using.
Heat a griddle or large nonstick skillet over medium heat and melt a little butter. Dip each brioche slice into custard for about 20 seconds per side, letting excess drip off. Cook until golden brown, 2-3 minutes per side.
Place a fried chicken thigh on one slice of French toast. Drizzle with warm maple syrup, dust with powdered sugar, and top with another slice of French toast. Serve immediately.