Prep: Preheat oven to 350°F. Generously spray a mini muffin pan with nonstick spray.
Dry Mix: Whisk flour, sugar, cocoa, baking soda, and salt in a large bowl.
Wet Mix: In a separate bowl, stir boiling water, butter, buttermilk, and vanilla until the butter is melted and the mixture is lukewarm.
Combine: Pour the wet mixture into the dry ingredients and stir. Fold in the beaten eggs until smooth and no flour streaks remain.
Bake: Use a 1 tbsp scoop to fill the mini muffin tins. Bake for 8–10 minutes until the tops spring back. Cool completely.
Setup: Place cooled bites on a wire rack over parchment paper (to catch the icing drips).
Make Icing: Microwave milk, butter, and vanilla for 25–30 seconds until melted. Stir in the cocoa powder, then whisk in the powdered sugar until smooth and glossy.
Glaze: Drizzle or dollop the warm icing over each bite. Let the icing set firmly before serving.