01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
02 - In a medium mixing bowl, sift together the all-purpose flour, cocoa powder, baking soda, and salt. Whisk gently 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 the mixture becomes light and fluffy, approximately 2 to 3 minutes. Add the egg and vanilla extract, continuing to beat until fully incorporated.
04 - Gradually add the sifted dry ingredients to the creamed butter mixture, mixing on low speed until a stiff, uniform dough forms. Avoid overmixing.
05 - Roll the dough into 1-inch balls and arrange them on the prepared baking sheets, spacing them about 2 inches apart. Gently flatten each ball using the palm of your hand or the bottom of a glass to about ¼-inch thickness.
06 - Bake for 10 to 12 minutes until the cookies are set and slightly firm to the touch. Allow them to rest on the baking sheet for 5 minutes before transferring to a wire cooling rack to cool completely.
07 - In a medium bowl, beat together the softened butter, vegetable shortening, sifted powdered sugar, vanilla extract, and a pinch of salt using an electric mixer on medium-high speed until the filling is smooth, creamy, and fluffy, about 2 minutes.
08 - Pipe or spread approximately 1 tablespoon of the vanilla 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 for 10 to 15 minutes at room temperature so the filling sets before serving.