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1
Preheat the oven to 350 degrees F. Arrange a rack in the upper third of the oven, and another in the bottom third.
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2
Line 2 baking sheets with parchment paper or silicone mats.
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3
In a large bowl, whisk together the flour, baking soda, baking powder, and salt.
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4
Set aside.
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5
Using a mixer fitted with the paddle attachment, cream the butter.
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6
Beat in the dark brown and granulated sugar for about 5 minutes or until very pale, light, and fluffy.
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7
Beat in the eggs one at a time, mixing well after each.
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8
Stir in the vanilla extract.
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9
Stir in the peanut butter.
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10
Reduce the mixer speed to low and add the flour mixture, beating until just combined, 5 to 10 seconds.
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11
Add the oats.
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12
Be careful not to overmix the dough.
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13
Scoop out 2 ounces of cookie dough and roll into balls with your hands.
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14
Using the bottom of a tumbler or other glass, flatten the cookie-dough balls into 1/2-inch high circles.
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15
Arrange the disks on the prepared baking sheets about 2 inches apart.
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16
Refrigerate for about 30 minutes.
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17
Using a mixer fitted with the paddle attachment, cream the butter with the peanut butter for about 5 minutes on medium speed.
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18
Reduce the speed to low and add the sifted powdered sugar, mixing until well combined.
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19
Scrape the filling into a large zipper-top or pastry bag and set aside.
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20
Bake the cookies until theyre a light golden brown, 8 to 10 minutes, rotating the baking sheets halfway through.
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21
Let the cookies rest on their baking sheets for 10 minutes, then transfer directly to wire racks to cool completely.
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22
Once the cookies have cooled completely, turn over half the cookies, flat side up.
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23
If youre using the zipper-top bag for piping, snip off a small tip of one of the bottom corners.
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24
Pipe about 1 tablespoon of the filling on cookies sitting flat side up.
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25
Place the remaining cookies on top and gently press to squeeze the filling out to the edges.
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26
Serve, or store in an airtight container at room temperature for up to 3 days.
