01 - Preheat the oven to 350°F and line two baking sheets with parchment paper.
02 - In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt. Set aside.
03 - In a separate bowl, beat the softened butter and granulated sugar together until light and fluffy. Add the egg and vanilla extract, beating until fully combined.
04 - Gradually add the sifted dry ingredients to the wet mixture, mixing until a stiff, uniform dough forms.
05 - Roll the dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. Flatten each ball slightly with the palm of your hand or the bottom of a glass.
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 rack to cool completely.
07 - In a bowl, beat together the softened butter, vegetable shortening, sifted powdered sugar, vanilla extract, and a pinch of salt until smooth and fluffy.
08 - Pipe or spread about 1 tablespoon of filling onto the flat side of half the cookies. Top with the remaining cookies to form sandwiches, pressing gently to spread the filling evenly to the edges.
09 - Allow the assembled cookies to set for 10 to 15 minutes at room temperature before serving.