Recipe adapted from Red Truck Bakery Cookbook by Brian Noyes (Clarkson Potter, 2018).
2¼ cups flour
½ teaspoon baking soda
½ teaspoon kosher salt
½ cup unsalted butter, melted
¾ cup unsweetened cocoa powder
2 cups sugar
¾ cup buttermilk
1½ teaspoons vanilla extract
2 cups semisweet chocolate chips
Preheat oven to 350 degrees. In a small bowl, whisk together flour, baking soda, and salt. In the bowl of a stand mixer fitted with paddle attachment, beat together melted butter, cocoa powder, and sugar on medium speed until well combined, about 2 minutes. Gradually add buttermilk and vanilla. Add flour mixture and beat until incorporated. Add chocolate chips and mix until just combined. Cover bowl with plastic wrap and chill dough for at least 30 minutes and up to 3 hours.
Portion out 2 rounded tablespoons of dough and, using your hands, form into a ball. Repeat with remaining dough and place onto two ungreased baking sheets, spacing balls 1½ inches apart. Flatten each ball slightly with your palm—they will continue to spread while baking. Bake for 8 minutes, turning baking sheets halfway through, until tops are puffy and slightly cracked. Transfer cookies to a wire rack to cool. Store in an airtight container or plastic bag for up to 3 days, or freeze in a plastic bag for up to 3 months.