From Classic Snacks Made From Scratch, by Casey Barber. Makes 3 dozen cookies.
3 c. unbleached all-purpose flour
½ c. unsweetened cocoa powder
1 tbsp. baked soda (see below)
1½ tsp. baking powder
1 tsp. kosher salt
2 oz. semisweet chocolate, coarsely chopped
8 tbsp. chilled unsalted butter, cut into ½ inch cubes
½ c. vegetable shortening
1 c. granulated sugar
2 large eggs
2½ c. powdered sugar
2 tbsp. light corn syrup
2 tbsp. whole or reduced-fat milk
1 tbsp. vegetable shortening
Make the cookies:
Whisk the flour, cocoa powder, baked soda, baking powder, and salt together in a large bowl, until no lumps remain.
Melt the chocolate in a small saucepan over low heat, stirring constantly, or microwave at 15-second intervals, stirring between each burst until liquefied. Set aside to cool slightly.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, shortening, and sugar on medium speed for 3-4 minutes, until light, fluffy and creamy. Reduce the speed to low and add the eggs one at a time, mixing throughly before adding the next. Pour in the melted chocolate and mix until just combined.
Add the dry ingredients and mix until a soft dough forms. Shape into 2 discs of roughly equal size and wrap each in plastic wrap. Refrigerate for at least 1 hour, until firm.
Preheat the oven to 375° F and line 2 baking sheets with parchment paper or Silpat liners.
On a floured work surface, roll one of the dough discs to ¼ inch thickness and stamp out cookie rounds, about 1½ inches in diameter. Transfer the rounds to the prepared baking sheet and repeat with the remaining disc of dough.
Bake for 10 to 12 minutes, or until the cookies are slightly crispy at the edges and puffed but still pliable in the centre. Cool completely on a wire rack before filling.
Fill the cookies:
Using the stand mixer with paddle attachment, mix the powdered sugar, corn syrup, milk, and shortening on low speed until combined, about 30 seconds. Increase the speed to medium and beat for 30 more seconds to whip slightly.
With the flat side of a cooled cookie facing up, spoon 1 tsp. of filing onto the centre. Top with a second cookie, flat side down, spreading the filling evenly.
Baked soda reacts with the cocoa powder during baking to bring out the signature bittersweet chocolate taste and near-black colour. Preheat the oven to 250° F. Cover a rimmed baking sheet with aluminium foil and spread a box of baking soda in an even layer on the foil. Bake for one hour and cool completely on the sheet. Store in a well-sealed glass jar and use within one year. Note that because of its concentrated alkaline content, baked soda is a mild irritant.