Preheat oven to 350°.
Remove white papery skin from garlic heads (do not peel or separate the cloves). Wrap each head separately in foil. Bake at 350° for 1 hour; cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins.
Heat olive oil in a small saucepan over medium heat. Add chopped prosciutto and coarsely ground black pepper; cook 5 minutes, stirring occasionally. Combine garlic pulp and oil mixture in a small bowl, stirring with a fork until well blended. Serve with bread slices. Garnish with parsley, if desired.