Preheat the oven to 350°F. Generously coat a 10-inch cast iron skillet with cooking spray. If you don’t have a cast iron skillet, grease a 10-inch round cake pan.
In a medium saucepan, combine the butter, peanut butter and sugars. Heat over medium-low heat until the butter and peanut butter are melted and the mixture can be stirred smooth. Do not boil. The mixture should be pourable and grainy from the sugar. Remove from heat and let cool until just warm. Whisk in the eggs and vanilla extract.
In a separate large bowl, combine the oats, flour, baking powder, baking soda and salt. Mix well. Pour the melted peanut butter mixture over the oat mixture. Using a rubber spatula, stir the mixture together, folding it together until no pockets of flour remain. Once the dough is consistent, add the candy-coated chocolates, chips and peanuts. Mix until well dispersed. Press dough into the pan evenly using a rubber spatula.
Bake for 25-30 minutes, or until the cookie is puffed and slightly brown on top. Remove from oven and let cool slightly. Serve slices warm with scoops of ice cream on top.