1. Preheat the oven to 375°F. Line two baking trays with parchment paper. Place the trays in the oven to warm right before making the cookie dough (this step will help the cookies bake with smooth curved tops).
2. For the cookies, sift the flour, sugar, cocoa powder, baking powder, baking soda, and salt into a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. Add the butter and mix on medium-low speed to cut it in until the pieces are barely visible.
3. In a separate bowl, whisk together the egg, milk, sour cream, and vanilla. Add to the flour mixture and mix on low speed to incorporate then increase to medium-high and beat for about 2 minutes, until the batter is fluffy and holds its structure. Use a medium ice cream scoop to scoop out 16–20 portions onto the now-hot, prepared baking trays, leaving 3 inches between them. Bake the "pies" for about 15 minutes, until the tops spring back when gently pressed. Cool the "pies" on the baking trays before filling.
4. For the filling, beat the butter to lighten it and then beat in the peanut butter. Add 1 cup of the icing sugar and beat in. Stir in the vanilla and then beat in the remaining 1 cup of icing sugar until fluffy. Fill a piping bag fitted with a large star tip, pipe filling onto one of the "pies," and top with a second "pie," pressing gently. Repeat with the remaining pies. They will keep in an airtight container, or on a plate covered in plastic wrap, for up to 3 days—no need to refrigerate.
Makes 8 to 10 large whoopie pies.
Excerpted from Bake With Anna Olson by Anna Olson. Copyright © 2016 Olson Food Concepts Inc. and Peace Point Entertainment Group Inc. Published by Appetite by Random House, a division of Penguin Random House Canada Limited. Reproduced by arrangement with the publisher. All rights reserved.