Preheat oven to 350 degrees Fahrenheit.

Whisk together flour, cocoa, baking powder, baking soda, and salt in a large bowl.

In another bowl whisk the eggs, white sugar, brown sugar, oil, orange zest, and vanilla together until smooth.

Pour half the wet ingredients and half the buttermilk into the dry mixture and gently whisk for a few seconds.

Add the rest of the buttermilk and wet mixture and switching to a wooden spoon, stir until just combined. (over stirring will result in thin batter)

Divide the batter evenly into 12 greased or lined muffin cups and bake for 18-20 minutes, or until toothpick comes out clean when stuck in the middle. Cool completely before frosting.

To make frosting, beat the butter until creamy, then add in powdered sugar 1 cup at a time. Scrape bowl edges, add in the orange and vanilla extract, and milk. Mix until fully combined.