Preheat oven to 350 degrees F (175 degrees C).
Mix crabmeat, Italian-style bread crumbs, panko, Parmesan cheese, mozzarella cheese, egg, milk, and butter in a bowl until thick yet pliable enough to shape. Divide and roll into 12 1-inch balls.
Gently press the balls into the mushroom caps and press to spread into a rounded mound atop the mushroom.
Bake in preheated oven until heated through and golden on top, about 30 minutes.
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