01 - Preheat oven to 350°F. Line two baking sheets with parchment paper and set aside.
02 - In a medium bowl, sift together the all-purpose flour, cocoa powder, baking soda, and salt. Whisk to combine evenly and set aside.
03 - In a large bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until light and fluffy, about 2–3 minutes. Add the egg and vanilla extract, continuing to beat until fully incorporated.
04 - Gradually add the sifted dry ingredients to the butter mixture, mixing on low speed until a stiff, uniform dough forms. Scrape down the sides of the bowl as needed.
05 - Roll tablespoon-sized portions of dough into 1-inch balls. Arrange them 2 inches apart on the prepared baking sheets. Flatten each ball slightly using the palm of your hand or the bottom of a drinking glass.
06 - Bake for 10–12 minutes until the cookies are set and slightly firm to the touch. Allow them to rest on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
07 - In a mixing bowl, beat together the softened butter, vegetable shortening, sifted powdered sugar, vanilla extract, and a pinch of salt on medium-high speed until smooth, light, and fluffy, about 2 minutes.
08 - Pipe or spread approximately 1 tablespoon of the cream filling onto the flat side of half the cooled cookies. Place the remaining cookies on top, flat side down, and press gently to distribute the filling evenly to the edges.
09 - Allow the assembled sandwich cookies to rest at room temperature for 10–15 minutes so the filling sets before serving.