Step 1

Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. (Always recommended for cookies.) Set aside.

Step 2

Whisk the flour, oats, baking soda, baking powder, and salt together until combined. Set aside.

Step 3

In a large bowl using a hand-held or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar together on high speed until completely smooth and creamy, about 2 minutes. Add the peanut butter, egg, and vanilla extract and beat on high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed to combine. Add the dry ingredients to the wet ingredients and mix on low until combined.

Step 4

Scoop cookie dough into balls, about 2 heaping teaspoons of dough per cookie (almost 1 Tablespoon– any size around here is great), and arrange 3 inches apart on the baking sheets. Using the back of a measuring cup or drinking glass, gently press down on each dough ball to slightly flatten. Bake for 10-11 minutes or until edges are lightly browned and appear set.

Step 5

Allow cookies to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely before sandwiching.

Step 6

In a large bowl using a hand-held or stand mixer fitted with a paddle attachment, beat the frosting ingredients together until combined and smooth. Spread the frosting onto the flat side of one cookie and sandwich with the other. (Or you can use a piping tip– I used piping tip #12 to pipe the frosting onto the cookies before sandwiching.) Repeat with remaining.

Step 7

Cookies will stay fresh covered at room temperature for 3 days or in the refrigerator for 1 week.