Homemade Oreo Cookies

Cookies

1 1/4 cups all-purpose flour
1/2 cup unsweetened cocoa powder
1 teaspoon baking soda
1/4 teaspoon baking powder
1/4 teaspoon salt
1 cup sugar
1/2 cup plus 2 tablespoons (1 1/4 sticks)

room-temperature, unsalted butter

1 large egg

Filling

1/4 cup (1/2 stick) room-temperature, unsalted butter
1/4 cup vegetable shortening (you could omit the shortening and use all butter, but the shortening helps the filling to set)
2 cups sifted confectioners sugar
1 Tbsp vanilla extract
a tiny pinch of salt

Set two racks in the middle of the oven. Preheat to 375°F.
In the bowl of an electric mixer, thoroughly mix the flour, cocoa, baking soda and powder, salt, and sugar. On low speed, add the butter, and then the egg. Continue processing or mixing until dough comes together in a mass.

Take rounded teaspoons of batter and place on a parchment paper-lined baking sheet approximately two inches apart. With a flat-bottomed glass, slightly flatten the dough by pressing down lightly. Bake for 9 minutes. Set baking sheets on a rack to cool.

To make the cream, place butter and shortening in a mixing bowl, and at low speed, gradually beat in the sugar, vanilla, and salt. Turn the mixer on high and beat for 2 to 3 minutes until filling is light and fluffy.
To assemble the cookies, in a pastry bag with a round tip, pipe the cream into the center of one cookie. Place another cookie, equal in size to the first, on top of the cream. Lightly press, to work the filling evenly to the outsides of the cookie. Continue this process until all the cookies have been sandwiched with cream