Preheat the oven to 350˚. Line two 12-cup muffin pans with paper liners.

Melt the butter in a medium saucepan over medium heat and stir in the cocoa powder. Pour in the boiling water and stir to combine. Let boil for 15 seconds, then remove from the heat and set aside.

In a medium bowl, stir together the flour, sugar and salt; set aside. In a small bowl, whisk together the eggs, buttermilk, vanilla and baking soda. Pour the cocoa mixture over the flour mixture and stir until halfway combined, then pour in the egg mixture and stir until the batter comes together.

Fill the muffin cups two-thirds full (a scant ¼ cup per cupcake) and bake until a toothpick inserted into the centers comes out clean, about 20 minutes. Remove from the oven and let cool in the pans for about 5 minutes, then remove to a rack and let cool completely before decorating.

Beat the butter in a medium bowl with a mixer on medium speed until creamy. Reduce the speed to low and add the confectioners’ sugar in two batches, beating until combined. Add the vanilla, increase the speed to high and beat for a good 30 seconds, until it’s light and creamy. Add 3 tablespoons milk and beat until fluffy, 2 to 3 minutes. Beat in the last tablespoon of milk if it seems too stiff. Beat in the chopped cookies until very well blended. Transfer to a piping bag fitted with a medium star tip.

Pipe the frosting onto the cupcakes and top each with a cookie piece.