Grease a 9-by-13-inch baking dish. Cook the bacon in a skillet until crisp, drain it, and crumble it.
Whisk the eggs, milk, salt, pepper, and garlic powder in a large bowl until smooth.
Scatter the bread cubes, half of the cheddar, and half of the bacon in the prepared dish.
Pour the egg mixture evenly over the bread, then press the cubes down gently so they are moistened. Top with the remaining cheddar and bacon.
Cover and refrigerate for at least 30 minutes or overnight.
Heat the oven to 350 degrees F. Bake uncovered for 40 to 45 minutes, until the center is set and the top is golden.
Rest the casserole for 10 minutes, then sprinkle with chives, slice, and serve warm.