Heat oven to 375ºF. Wipe mushrooms gently with a damp paper towel. Remove and finely chop stems. In a large skillet over medium heat, melt 3 tablespoons of the butter. Add mushroom stems and cook for 5 minutes or until tender. Remove from heat and stir in 1/4 cup of the panko, the shredded crab, onion powder, salt and pepper.
Stuff mushrooms caps with crab mixture and place them stuffing side up on an ungreased rimmed baking sheet. In a small bowl, combine remaining butter and panko. Sprinkle evenly over the tops of the mushrooms and bake for 15 minutes.