01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - Sift together the flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
03 - In a separate bowl, beat the butter and granulated sugar together until light and fluffy. Add the egg and vanilla extract, beating until fully incorporated.
04 - Gradually add the dry ingredient mixture to the wet ingredients, 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 using 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 cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
07 - Beat together the butter, vegetable shortening, powdered sugar, vanilla extract, and salt in a bowl 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, flat side down, to create 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.