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, Dutch-process 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 pale, light, and fluffy, about 2 to 3 minutes.
04 - Add the egg and vanilla extract to the butter mixture. Continue beating until fully incorporated and smooth.
05 - Gradually add the sifted dry ingredients to the wet mixture, mixing on low speed until a stiff, cohesive dough forms. Do not overmix.
06 - Roll tablespoon-sized portions of dough into 1-inch balls. Arrange them on the prepared baking sheets, spacing each about 2 inches apart. Gently flatten each ball with the palm of your hand or the bottom of a glass to about ¼-inch thickness.
07 - Bake for 10 to 12 minutes, or until the cookies are set and slightly firm to the touch. Allow them to rest on the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely.
08 - 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.
09 - 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 to the edges.
10 - Allow the assembled sandwich cookies to rest at room temperature for 10 to 15 minutes so the filling sets before serving.