In a large skillet over medium low heat, melt the butter and peanut butter, stirring until smooth. Stir in the marshmallows and cook until melted, stirring constantly.
Remove from heat and toss with Cheerios. Wait a moment and add the Reese's minis to combine. (Wouldn't want them to melt from the heat if added right away).
Quickly press into the prepared baking dish and allow to set.
Meanwhile, melt the chocolate and drizzle over top.