01 - Preheat the oven to 350°F and line two baking sheets with parchment paper.
02 - Sift together the all-purpose flour, cocoa powder, baking soda, and salt in a medium bowl. 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 incorporated.
04 - Gradually add the sifted dry ingredients to the butter mixture, mixing until a stiff, cohesive 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 cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
07 - Beat together the softened butter, vegetable shortening, sifted powdered sugar, vanilla extract, and salt in a bowl until smooth and fluffy.
08 - Pipe or spread about 1 tablespoon of the cream filling onto the flat side of half the cookies. Top with the remaining cookies, flat side down, to form sandwiches. Press 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.