In the bowl of a stand mixer fitted with the whisk attachment, combine all filling ingredients. Mix on medium speed until creamy and fully combined. Transfer mixture to another bowl; cover and refrigerate.

Clean and dry stand mixer bowl.

Add flour, baking soda, baking powder and salt to a medium bowl; whisk to combine. Whisk in the oats. Set aside.

Add butter, chunky peanut butter, and sugars to the bowl of the stand mixer fitted with the paddle attachment. Mix at medium-high speed until pale and fluffy, 3 to 4 minutes, occasionally scraping down the bowl.

Reduce mixer speed to medium-low and add eggs, one at a time, until combined. Scrape down bowl and add vanilla extract; mix until combined. Add dry ingredients in 2 or 3 additions and mix just until combined. Remove bowl from mixer and scrape down sides with a rubber spatula, being sure to incorporate any dry ingredients still in the bottom.

Line a baking sheet with parchment paper.

You'll be chilling these scoops, so place them close together (not touching). Scoop out dough with a 1-tablespoon scoop. Place scoops on prepared baking sheet. You'll have about 48 cookies. Refrigerate scooped cookies at least 2 hours.

Line baking sheet with parchment paper and create a double pan by setting it inside a second baking sheet.

Arrange 8 to 12 cookies on prepared baking sheet (number depending on the size of your sheet) , leaving an inch or more of space on all sides of each cookie (cookies will spread during baking). Bake until golden, about 12 minutes, rotating pan halfway through the cooking time. When golden, remove from oven and cool on wire rack 10 minutes before transferring cookies to cooling rack with a spatula. Repeat with remaining cookies.

Allow the cookies to cool completely before filling them.

Turn one cookie bottom side up and scoop a little less than 2 teaspoons of peanut butter filling into the middle. Top with another cookie, bottom side down, and press gently to spread filling. Repeat to assemble remaining sandwich cookies.