1. Line a baking sheet with parchment paper, set aside. Melt 4 tablespoons butter in a medium saucepan over medium heat. Add oats and cook, stirring, until toasted, 5 to 7 minutes. Spread oat mixture onto prepared baking sheet and let cool.
2. Meanwhile, preheat oven to 350 degrees. Line 2 baking sheets with parchment paper or a nonstick baking mat; set aside.
3. In a medium bowl, whisk together flour, baking soda, and salt; set aside. In the bowl of an electric mixer fitted with a paddle attachment, cream together remaining ½ cup butter, granulated and brown sugars. Add peanut butter and mix until well combined.
4. Add oat mixture and flour mixture; mix on low speed until combined. Turn dough out onto a piece of parchment paper. Cover dough with second piece of parchment paper and roll out dough, between parchment, to ¼ inch thick. Carefully slide rolled dough onto the back of a baking sheet and transfer to a refrigerator until chilled, about 20 minutes.
5. Remove top layer of parchment paper; cut out cookies using a 2-inch round cookie cutter. Place cookies 1 inch apart on prepared baking sheets; sprinkle with Morena sugar.
6. Transfer baking sheets to oven and bake until golden, rotating baking sheets halfway through baking, about 10 minutes. Let cool completely on baking sheets.
7. Mix all remaining ingredients together in the bowl of an electric mixer fitted with the paddle attachment on medium speed until smooth. Transfer filling to a disposable pastry bag until ready to use.
8. When cookies have cooled completely, snip the end of the pastry bag and squeeze filling in a spiral motion on the bottom side of half of the cookies. Top with remaining cookies to form sandwiches.
Recipe By: Leslie Reese