Arrange rack to center of oven and preheat oven to 450F (235C) and line several baking sheets with parchment paper. Set aside.

In a large mixing bowl, combine sugar, butter, egg, and vanilla extract and beat until pale yellow and well-combined.

Add buttermilk and stir until combined.

In a separate bowl, whisk together flour, cocoa, baking soda, baking powder, and salt.

With mixer on low-speed, alternate adding the flour mixture and the hot water (be careful not to splash yourself!) to the butter/sugar mixture, beginning and ending with the flour mixture and stirring after each addition (I add the flour in 3 parts and the water in 2). Stir until batter is smooth and combined (it will be thin like a cake batter).

Drop cooking batter by 1 ½ Tablespoon-sized scoops onto prepared baking sheet, spacing at least 2” apart.

Bake for 5 minutes in center rack of oven then remove and allow to cool on baking sheet for at least 10 minutes before carefully removing to cooling rack to cool completely before filling.

Cookie Dough Filling

Heat treat your flour: Place flour in heap in a microwave safe dish and heat in 10-second intervals until an instant read thermometer inserted into the heap registers at least 160F (72C). Allow flour to cool before proceeding.

Combine butter, sugars, vanilla extract and salt and use an electric mixer to beat together until creamy and well-combined.

Gradually add flour until completely combined.

With mixer on low speed, slowly add milk until the dough reaches a spreadable consistency.

Fold in chocolate chips.

Assembly

Pair off chocolate cookies of similar sizes.

Scoop, spread, or pipe cookie dough filling onto the bottom of one cookie, then sandwich with another similar-sized cookie. Repeat until all cookies have been filled.