Preheat oven to 425°F. Line a large baking sheet with parchment paper.
Remove outer leaves from cauliflower and trim the stem end flat. Place cauliflower stem-side down and carefully slice through the center into 1-inch thick steaks. You should get 2-3 steaks per head, plus florets on the sides.
Brush both sides of cauliflower steaks with olive oil and melted butter. In a small bowl, mix together garlic powder, smoked paprika, salt, and pepper. Sprinkle seasoning mixture evenly over both sides of the steaks.
Arrange cauliflower steaks on the prepared baking sheet. Roast for 25-30 minutes, flipping halfway through, until golden brown and tender when pierced with a fork.
Remove from oven and top each steak with shredded cheddar cheese and crumbled bacon. Return to oven for 3-5 minutes until cheese is melted and bubbly.
Transfer cauliflower steaks to serving plates. Top each with a dollop of sour cream and a generous sprinkle of fresh chives. Serve immediately while hot and cheesy.