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1
In a bowl, whisk together flour, baking soda, and salt; set aside.
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2
Place butter, peanut butter, and both sugars in the bowl of an electric mixer fitted with the paddle attachment; beat on medium-high speed until light and fluffy, 2 to 3 minutes.
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3
Add the egg and vanilla, and beat to combine, scraping down the sides of the bowl.
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4
With mixer on low, add reserved flour mixture and beat until incorporated, about 1 minute.
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5
Divide dough in half, and shape into two flattened disks.
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6
Wrap in plastic and refrigerate at least 30 minutes or overnight.
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7
Preheat the oven to 325F.
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8
Line two large baking sheets with parchment paper.
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9
(If refrigerated overnight, let dough stand at room temperature to soften slightly.)
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10
On a lightly floured work surface, roll out dough to a scant 1/4-inch thickness.
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11
Using a sharp knife, cut dough into 2 1/2-by-1-inch rectangles.
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12
Using the floured tines of a fork, score the top of each cookie.
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13
Transfer cookies to prepared baking sheets, placing 1 1/2 inches apart, and refrigerate until firm, about 15 minutes.
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14
Bake, rotating sheets halfway through, until lightly golden around the edges and firm in the center, 15 to 20 minutes.
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15
Transfer to a wire rack to cool completely.
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16
Using an offset spatula, spread 1 tablespoon filling onto the flat sides of half of the cookies.
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17
Sandwich with remaining cookies, keeping the flat sides down.
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18
Unfilled cookies can be kept in an airtight container at room temperature for up to 1 week.
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19
Once filled, cookies are best eaten the same day, but can be kept in the refrigerator for up to 3 days.
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20
In the bowl of an electric mixer fitted with the paddle attachment, combine all ingredients.
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21
Beat on medium-high speed until light and fluffy, about 5 minutes, scraping down the sides of the bowl as needed.
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22
Use immediately, or transfer to an airtight container and refrigerate up to 3 days.
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23
If refrigerated, let filling stand at room temperature to soften before using.
