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1
Line 2 baking sheets with parchment paper and spray with cooking spray.
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2
Beat brown sugar, butter, and white sugar together in a bowl using an electric mixer until smooth and creamy, about 5 minutes. Scrape down sides of bowl a few times. Continue beating mixture while adding eggs, 1 at a time, beating well after each addition. Add vanilla extract and mix well.
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3
Whisk flour, baking soda, and salt together in a separate bowl. Slowly add flour mixture to creamed butter mixture while beating on low speed until dough is just mixed. Fold oats and butterscotch chips into dough.
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4
Scoop 32 balls of dough using a large cookie scoop and arrange on the prepared baking sheets. Spray the bottom of a broad, flat glass or bowl with cooking spray and press onto each dough ball to flatten. Place a chocolate sandwich cookie on top of 16 flattened dough balls and cover with remaining 16 flattened dough balls, sealing the edges together with your fingers.
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5
Transfer stuffed cookies to a container or tray and cover with a lid or plastic wrap. Refrigerate at least 1 hour to overnight.
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6
Preheat oven to 375 degrees F (190 degrees C). Line baking sheets with parchment paper.
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7
Arrange cookies on the sheets with no more than 6 per sheet; top with a sprinkling of flaked salt.
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8
Bake 1 tray at a time in the preheated oven until golden brown, 14 to 16 minutes. Cool on the baking sheet for 1 minute before transferring to a wire rack to cool.
