In a bowl, mix butter and brown sugar until smooth. Stir in flour, oats, peanuts, and coconut until well blended.
Press dough evenly into a buttered and floured 10- by 15-inch baking pan. For chewy bars, bake in a 350° oven until edges are lightly browned, about 20 minutes; for crisp bars, bake until slightly darker brown, about 5 minutes longer. Spread with peanut butter. Let cool about 15 minutes.
Seal chocolate chips in a heavy 1-quart zip-lock plastic bag. Heat in a microwave oven at 30% power until chips are soft, about 2 minutes. Squeeze chocolate to one corner of bag, then use scissors to snip a very small hole in the corner. Squeeze chocolate in thin lines over peanut butter. Chill until the chocolate is firm, about 20 minutes. Cut into 20 bars.