In a stand mixer, cream the butter and sugar together until fluffy. Beat in the eggs one at a time.

In a separate bowl, whisk together the flour, salt, cocoa and baking soda. Slowly beat the flour mixture into the butter mixture until well combined. Form the dough into 2 small disks and wrap with plastic wrap. Chill for 30 minutes in the refrigerator.

Make the filling - in a stand mixer, beat the butter until fluffy. Add the confectioner's sugar and beat until combined. Beat in the vanilla until creamy.

Preheat the oven to 350 degrees. Roll the dough onto a floured surface until it is about 1/4 inch thick. Using a 2-inch biscuit cutter, cut out the cookies and place them onto a parchment-lined baking sheet. Use a cookie stamp or fork to make designs in the cookies by lightly pressing down so that you don't flatten them too much. Bake 12-14 minutes.

Allow cookies to cool completely before assembling. To assemble, pipe frosting in a circle on the inside of one cookie. Fill in the circle with additional frosting. Gently press another cookie down onto the filling until a sandwich is formed.