for the chocolate cookies
1 1/4 cups all-purpose flour
1/2 cup unsweetened Dutch process cocoa
1 teaspoon baking soda
1/4 teaspoon baking powder
1/4 teaspoon salt
1 cup plus 2 tablespoons sugar*
1/2 cup plus 2 tablespoons (1 1/4 sticks) unsalted butter, at room temperature
1 large egg
for the frosting
1/4 cup (1/2 stick) unsalted butter, at room-temperature
1/4 cup non-hydrogenated vegetable shortening
2 cups confectioners’ sugar, sifted
2 teaspoons vanilla extract
Place oven racks in the middle two racks. Preheat oven to 375*F and line two baking sheets with parchment.
In a large bowl, combine the flour, cocoa, baking soda and powder, salt, and sugar; stir to combine. Using a mixer (hand or stand) on low speed, add the butter in tablespoons, then add the egg until the dough comes together.
Form small balls of dough with a teaspoon and flatten them a bit with the palms of your hands. Place on the baking sheets (about 16-20 per sheet, depending on the size of your cookies). Bake for 9 minutes, rotating sheets halfway through. Set sheets on a rack to cool. When cool.
To make the filling, beat butter and shortening to combine. Add sugar, 1/2 cup at a time, until blended. Add vanilla, mix to blend. Increase speed to high and whip for 3 minutes until fluffy.
Transfer frosting to piping bag fitted with round tip. Pipe filling onto middle of the flat side of a cookie and match it up with a same-size cookie; press together slightly to distribute the frosting.
Store in airtight container up to a week.
adapted from Retro Desserts via Smitten Kitchen
*After reading Deb's comments, I decreased the amount of sugar used to better replicate the balance of the traditional Oreo. I knew I would be making a few sandwich cookies with a lot of filling (I'm a frosting junkie), so the more bitter chocolate cookie was important to me. I'm happy with the balance I struck.