Type of candy bars: I used a combination of peanut butter cups, snickers, twix, and kitkats for the batter, and topped with even more of these plus reese's pieces after baking.
Top with extra candy: I recommend waiting to put more candy on top until after baking so they don't fully melt in the oven. They'll melt the perfect amount if you put them on top right after they come out of the oven!