Melt the butter in a saucepan or the microwave in a microwave-safe bowl. Mix in the sugar, eggs, and vanilla. Then, mix in the cocoa powder, all-purpose flour, salt, and baking powder. Fold in the Hershey's bars, graham cracker pieces, and marshmallows (break these up into chunks/bits). Save more of each mix-in to put on top after the brownies come out of the oven.