01 - Preheat the 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 mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until light and fluffy, about 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 glass dipped in cocoa powder.
06 - Bake for 10 to 12 minutes until the cookies are set and slightly firm to the touch. Let them rest on the baking sheet for 5 minutes, then transfer to a wire cooling 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 the filling is smooth, creamy, and fluffy, about 2 minutes.
08 - Pipe or spread approximately 1 tablespoon of filling onto the flat side of half the cooled cookies. Top each with a second cookie, flat side down, and press gently to distribute the filling evenly toward the edges.
09 - Allow the assembled sandwich cookies to rest at room temperature for 10 to 15 minutes so the filling firms up before serving.