Step 1In the bowl of a stand mixer, or in a large bowl using an electric mixer, cream together the butter and sugar until light and fluffy, 3 to 5 minutes. Add the eggs, 1 at a time, until fully incorporated. Beat in the vanilla.
Step 2In a large bowl, sift together the flour, cocoa powder, baking soda and salt.
Step 3Beat in the dry ingredients, one spoonful at a time, until incorporated to form the cookie dough. Scrape the bowl partway through to make sure the ingredients are fully mixed. Cover and refrigerate the dough until chilled.
Step 4Heat the oven to 375 degrees. Scoop the dough into tablespoon-sized pieces, and place on parchment-lined baking sheets. Flatten each cookie slightly, and bake each sheet until the cookies are set, 10 to 12 minutes.
Step 5Remove and cool before filling. This makes about 5 dozen individual cookies.
Step 1In the bowl of a stand mixer, or in a medium bowl using an electric mixer, cream together the butter and sugar until light and fluffy. Beat in the vanilla, water and salt. Scoop 1 generous tablespoon of filling onto one cookie, then top with a second, pressing together until the filling comes to the edge. Repeat to make 2 1/2 dozen sandwich cookies. You might have a little frosting left over; extra frosting will keep for 1 week, covered and refrigerated.