Peanut Butter Sandwich Cookies (adapted slightly from The Brown-Eyed Baker)
2 cups all-purpose flour, plus more for dusting
½ teaspoon baking soda
¼ teaspoon salt
¾ cup unsalted butter, at room temperature
¾ cup creamy peanut butter
½ cup granulated sugar
½ cup brown sugar
1 egg
1 teaspoon vanilla
Peanut Butter Filling:
6 tablespoons unsalted butter, at room temperature
1/2 cup powdered sugar
¾ cup creamy peanut butter
3 tablespoons heavy cream
In a medium bowl, whisk together the flour, baking soda, and salt; set aside.
Beat the butter, peanut butter, and sugars until light and fluffy, 2 to 3 minutes. Add the egg and vanilla and beat to combine, scraping down the sides of the bowl as needed. Add the flour mixture, mixing until incorporated, about 1 minute.
Divide the dough in half, and shape into two rectangles. Wrap in plastic (or put in plastic bags) and refrigerate at least 30 minutes or up to 1 day.
When ready to bake the cookies, preheat the oven to 325 degrees. Line two baking sheets with parchment paper. If you refrigerated the dough longer than 30 minutes let it stand at room temperature to soften a bit. On a sheet of waxed paper, sprinkle the dough with a teeny bit of flour and roll out to a ¼-inch thickness. Using a sharp knife or pizza cutter (I used a pizza, cutter. Awesome!), cut the dough into 2 inch-by-1 inch (ish) rectangles. Using a fork, press the top of each cookie. Transfer cookies to prepared baking sheets, placing about an inch apart, and refrigerate about 15 minutes.
Bake until lightly golden around the edges, 15 to 20 minutes. Transfer to a wire rack to cool completely.
Meanwhile, make the peanut butter filling. Combine the butter, powdered sugar, peanut butter, and heavy cream and beat until light and fluffy, about 5 minutes, scraping the sides of the bowl as needed. (This also helps the flavors meld, otherwise it just tastes like cream and peanut butter) Use immediately, or store in an airtight container and refrigerate up to 3 days.
Spread filling onto the flat sides the cookies, as much or as little as you want! (You can frost half of them first, I just built a sandwich at a time.) Unfilled cookies can be kept in an airtight container at room temperature for up to 1 week. Once filled, cookies are best eaten the same day, but can be kept in the refrigerator for up to 3 days.