Preheat oven to 350 degrees. In a bowl, mix together the flour, baking powder and baking soda; set aside. Using a mixer with a paddle attachment, cream together the butter and peanut butter. Add sugars and beat at medium speed for 4 minutes, scraping down bowl twice.

At low speed, add eggs and vanilla. Add flour mixture and beat at low speed until well mixed, frequently scraping down bowl. Add peanuts and oats, and mix well. Using an ice cream scoop 2 inches in diameter, place balls of dough on parchment-lined baking sheets at least three inches apart.

Bake until cookies have spread and turned very light golden brown, about 10 minutes. Remove from oven and set aside to cool and firm up, 5 to 10 minutes. Transfer to a rack to cool completely before filling.

Using an electric mixer, cream together the butter, peanut butter, and confectioners’ sugar until very smooth. Transfer the filling to a pastry bag or makeshift pastry bag (i.e. Ziploc bag). To assemble cookies, pipe filling on underside of a cookie. Sandwich with another cookie. Repeat.