In a medium bowl, whisk together 1 3/4 cups plus 1 1/2 tablespoons all-purpose flour, 1 cup plus 1 1/2 tablespoons cocoa powder, and 3/8 teaspoon baking soda. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, beat 2 sticks (8 ounces) unsalted butter on medium-low speed until smooth. Add 1 teaspoon kosher salt and continue to mix for another 15 to 30 seconds. Add 3/4 cup plus 1 tablespoon granulated sugar and mix for about 2 minutes, until fluffy. Use a rubber spatula to scrape down the sides and bottom of the bowl before adding the dry ingredients (from the first step) in 2 additions, mixing on low speed for 15 to 30 seconds after each, or until just combined. Be careful not to overmix, and just mix until the dough has come together!
Mound the dough on a work surface lined with wax paper and push it together until it forms a 6-inch square block. Wrap in plastic wrap and refrigerate for at least an hour, or until firm.
When the dough is chilled and you're ready to bake, position a rack in the center of the oven and preheat the oven to 325 (F). Prepare a pan by lining it with parchment paper or a Silpat mat.
Unwrap the dough and place it between two pieces of wax paper. With a rolling pin, pound the top of the dough, working from left to right, and begin to flatten it. Turn the dough 90 degrees and repeat — this will help prevent the dough from cracking as it is rolled. Roll out to a 1/8-inch thick sheet. Be careful not to overwork the dough and let it thaw too much, otherwise it'll be too hard to work with! If it gets too melty, simply refrigerate it until it's firm enough to cut.
Using a 2 1/2-inch fluted cutter, cut rounds from the dough. If necessary, push the trimmings together and reroll. I got around 60 cookie cuts, which translates to 30 sandwich cookies. Arrange the rounds on your prepared sheet pan, leaving about 3/4 inch between them. Bake in the preheated oven for around 12 - 14 minutes, or until the cookies are fragrant with small cracks on the surface. Set the pans on a cooling rack and allow them to cool completely.