Preheat the oven to 375°F and prepare a baking sheet lined with foil to hold the bread while baking.
Use a serrated knife to cut the bread diagonally into slices without cutting all the way through, then turn and repeat to create a grid pattern.
Melt the butter in a small saucepan and stir in the garlic powder until fully combined and fragrant.
Use a pastry brush to coat the inside of each cut with garlic butter, then fill the gaps with mozzarella, Parmesan, and chopped parsley.
Wrap the loaf securely in foil and place it on the prepared baking sheet to keep everything contained while baking.
Bake the wrapped loaf until the cheese melts and the bread is heated through, then open the foil and continue baking to lightly crisp the top.
Remove from the oven and serve immediately with marinara sauce on the side for dipping.