In a large mixing bowl, add the peanut butter, the softened butter, the brown sugar, and the granulated sugar and combine using an electric mixer until smooth.
Add the eggs and the vanilla to the peanut butter mixture and mix until combined.
Add the flour, the baking powder, and the salt and use the electric mixer on low speed to mix the dry ingredients into the peanut butter mixture until a dough forms.
Add the marshmallows and the candy pieces and fold into the dough using a rubber spatula.
Pour the dough onto the prepared cookie sheet, spreading it out evenly.
Bake until the top of the cookie is just starting to brown, about 18-20 minutes.
Allow the cookies to cool slightly before slicing and serving.