E.l. Fudge-Style Chocolate and Vanilla Sandwich Cookies

Ingredients

• For the cookies:
• 3 1/2 ounces sugar
• 1/4 teaspoon Diamond Crystal kosher salt; for table salt, use about half as much by volume or use the same weight
• 1/2 teaspoon baking powder
• 4 ounces unsalted butter, pliable but cool
• 1 ounce milk, any percentage will do
• 1/2 ounce vanilla extract
• 7 3/4 ounces all-purpose flour, plus more for dusting
• For the filling:
• 4 ounces powdered sugar
• 1 ounce Dutch-process cocoa powder, such as Cacao Barry Extra Brute
• 3 ounces unsalted butter, brought to about 72°F
• 1/4 teaspoon vanilla extract
• 1/8 teaspoon Diamond Crystal kosher salt; for table salt, use about half as much by volume or use the same weight

Directions

1. For the cookies, set an oven rack in the lower-middle position and heat the oven to 350 degrees F (177 degrees C). Add the sugar, salt, baking powder, and butter to the bowl of a stand mixer with the paddle attached. Start on low just until everything is moistened, then raise the speed to medium and beat until the mixture looks pale, soft, and fluffy, about 5 minutes. Use a flexible spatula to clean down the bowl and paddle, then turn the mixer back to low. Pour in about half of the milk and vanilla; when that has blended in, add about half of the flour. Do the same with the remaining liquid and dry ingredients, then scrape the bowl and paddle again so the dough is evenly mixed.
2. Scatter plenty of flour over your work surface and roll the dough to 1/8-inch thick, adding more flour as you need it so it does not stick. Brush off any extra flour with a pastry brush, and use a docking tool to decorate the dough if you like. Stamp out cookies with a cutter that averages about 2 1/2 inches, then move them to a parchment-lined half sheet pan. Press the scraps together briefly, roll them again, and cut more cookies the same way. Bake until the cookies feel set and are lightly browned across the surface, about 16 minutes; rotate the pan midway through if that helps them color evenly. Leave the cookies to cool on the pan, then move on to the filling, or pack them in an airtight container and hold them at room temperature for up to 3 days.
3. For the filling, sift the powdered sugar and cocoa powder into the same bowl. In the bowl of a stand mixer fitted with the paddle, combine the softened butter, vanilla, and salt with about 1/3 of the sugar-cocoa mixture. Mix on low until the dry ingredients are dampened, then add another 1/3 of the sugar and cocoa and gradually bring the mixer up to medium. After that portion is mixed in, lower the speed and add the rest. Increase the speed slowly to medium again, stop to scrape the bowl and paddle with a flexible spatula, then continue mixing until the filling is smooth and creamy. If the butter starts out too cold, the filling can look thick or stubborn, so soft butter is key here. Spoon the filling into a disposable pastry bag and cut the tip to make a 1/4-inch opening.
4. To assemble, turn half of the cookies over and pipe about 1/4 ounce of filling onto each one, judging the amount by sight rather than measuring. After half of the cookies are filled, move any extra filling around where it is needed. Cap them with the remaining cookies and place the sandwich cookies in an airtight container.
5. Chill the cookies for 15 minutes so the filling can firm up. If your kitchen often gets warmer than 70 degrees F (21 degrees C), store the cookies in the refrigerator and bring out a few at a time to warm up before serving. Kept that way, they will last about 1 month. If your kitchen usually stays below 70 degrees F, chill them first, then store them at room temperature for up to 1 week.