In a medium saucepan over medium heat, cook and stir the garlic in the butter until tender.
In a medium bowl, mix garlic and butter, chili powder, 3/4 cup Parmesan cheese, mayonnaise, Italian-style breadcrumbs, salt and pepper. Increase the amount of mayonnaise, if desired. Fold in the crabmeat.
Stuff mushroom caps generously with the mixture from the bowl.
Pour cooking sherry in a large baking dish. Arrange stuffed mushroom caps, stuffing side up, in the baking dish. Top with remaining Parmesan cheese. Bake 10 minutes in the preheated oven.
Raise oven temperature to broil. Continue cooking mushrooms 3 to 5 minutes, or until stuffing is bubbly and lightly browned. Allow to cool slightly before serving.