What you need:
For the oreo cookies:
1 and 1/4 cups (160g) all-purpose flour (careful not to overmeasure)
1/2 cup (43g) unsweetened natural cocoa powder
1 teaspoon baking soda
1/8 teaspoon salt
1/2 cup (115g) unsalted butter, softened to room temperature
3/4 cup (150g) granulated sugar
1/4 cup (50g) packed light brown sugar
1 large egg, at room temperature
1 teaspoon vanilla extract
For the filling:
1/4 cup (60g) unsalted butter, softened to room temperature
1/4 cup (48g) vegetable shortening, room temperature
1 and 3/4 cups (210g) confectioners' sugar
1 teaspoon vanilla extract
What to do:
Make the cookies:
Toss the flour, cocoa powder, baking soda, and salt together in a large bowl. Set aside.
Using a handheld or stand mixer with a paddle attachment, beat the butter on high speed until creamy, about 1 minute. Scrape down the sides and the bottom of the bowl as needed. Switch the mixer to medium speed and beat in the granulated sugar and brown sugar until combined. Beat in the egg and vanilla extract, scraping down the sides and bottom of the bowl as needed.
Turn the mixer off and pour the dry ingredients into the wet ingredients
Turn the mixer on low and slowly beat until a dough is formed. Cover the dough tightly with aluminum foil or plastic wrap and chill for 30 minutes. Chilling is mandatory.
Preheat oven to 350F degrees. Remove the cookie dough from the refrigerator. Line two large baking sheets with parchment paper or silicone baking mats. Set aside.
Roll the dough into small balls, 2 teaspoons of slightly chilled dough per ball, and place 12 on each baking sheet. Press down on the balls to slightly flatten as pictured above. Bake each batch for 6-8 minutes. These cookies are small and bake very quickly. They will appear soft when done. Remove from the oven and allow to cool for at least 5 minutes on the baking sheet before transferring to a wire rack to cool completely. You'll make 3-4 batches (38-40 cookies total).
While the cookies are cooling, make the cream filling:
In a small bowl using a handheld or stand mixer with a paddle attachment, beat the butter and shortening together on high speed until creamy and combined, about 1 minute. Turn the mixer off and add the confectioners' sugar and vanilla. Beat on low for 1 minute and then switch to high and beat for 1 more minute until creamy and combined. The cream filling is thick.
I split the icing into two and one I folded in melted chocolate.
Spread cream filling between two cooled chocolate cookies. Repeat with the rest. Cookies stay fresh covered at room temperature for up to 1 week. Cookies freeze well, up to 2 months. Cream filling may be made ahead of time (up to 2 days in advance). Cover and chill when storing. Allow to come to room temperature before using.
Recipe from: Sally's Baking Addiction