Preheat the oven to 375 degrees F.
Place the slices of ciabatta on a parchment lined baking sheet. Bake until crisp and golden around the edges, about 8 minutes. Remove the bread from the oven. Place the prosciutto slices and mozzarella on the crostini toast and return to the oven to melt the cheese, about 8 more minutes.
Meanwhile combine the butter and the sage leaves in a small saucepan over medium heat. Cook until the butter is melted and starting to brown in spots and the sage leaves are crisp, about 5 minutes. Add the salt and pepper.
Remove the baking sheet from the oven. Transfer the crostini to a serving plate. Drizzle the crostini with the sage butter and serve immediately.