Preheat oven to broil on high. Slice loaf of bread in half horizontally (crusts down) and place both halves on a large sheet pan (line with foil, parchment paper, or spay with cooking spray). Set aside.
In a bowl, mix softened butter and garlic together until smooth. In a separate large bowl, mix together the cheeses, mayo, and sour cream until combined. Combine the two mixtures.
Spread and smooth cheese mixture evenly on both loaves. Place cheesy bread under the broiler for 2–4 minutes or until nicely browned. Watch carefully to avoid burning!
Meanwhile, cut pepperoni in quarters to resemble finger nails.
Remove and allow to slightly cool before cutting (use a sharp bread knife) into thin slices to resemble “fingers.” Place one pepperoni wedge at the top of each thin slice.
Enjoy immediately with a side of marinara sauce if desired.