Combine softened butter, garlic, parsley, olive oil, Parmesan cheese, garlic powder, salt, and black pepper in a bowl until fully mixed and creamy. Mixing thoroughly ensures the garlic flavor spreads evenly throughout the bread.
Slice the baguette lengthwise and place both halves on a parchment-lined baking tray with the cut side facing upward. This positioning helps the bread crisp properly while holding the butter mixture in place.
Spread the garlic butter evenly over the bread and top with mozzarella cheese if desired. Covering the bread evenly prevents dry areas and creates consistent flavor in every slice.
Bake at 400°F for 10–12 minutes until the edges become golden and crisp. Broil briefly at the end for deeper color, but watch closely to avoid burning the garlic.
Allow the bread to rest for 2 minutes before slicing and serving warm. This short cooling period helps the butter settle into the bread and improves texture.