Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Lightly grease a mini muffin pan.

Whisk the flour, sugar, cocoa, baking powder, and salt in the bowl of an electric mixer. Gradually add buttermilk, eggs, and oil, mixing until just combined. Do not overmix. Fold in chocolate and nuts.

Allow the batter to rest for 30 minutes; it will be slightly thicker than typical muffin batter.

Spoon the batter into the prepared muffin pan, filling each cup three-quarters full. Bake for 13 minutes, or until a toothpick inserted in the center comes out clean. Cool slightly before removing the muffins from the pan, then allow to cool completely.

Beat the butter in an electric mixer until light and fluffy. Add the remaining ingredients and mix until smooth.

Transfer the frosting to a piping bag. Pierce the top of each muffin slightly with the tip and pipe the frosting into the center, finishing with a swirl on top.

Garnish with crushed toasted macadamia nuts and chocolate shards.