Preheat oven to 350°F (175°C). Line a 12-cup muffin tin.
Whisk: 1½ cups flour, ¾ cup cocoa, 1 tsp baking powder, ½ tsp baking soda, ¼ tsp salt.
Beat ½ cup butter with ¾ cup sugar + ½ cup brown sugar until fluffy.
Beat in 2 eggs, 1 tsp vanilla, 1 tsp orange extract (optional), and zest of 1 orange.
Mix in dry ingredients alternately with ½ cup buttermilk and ½ cup fresh orange juice (start and end with dry). Mix just until combined.
Fill liners ⅔ full. Bake 18–22 min. Cool completely.
Beat 1 cup butter. Add 3½ cups powdered sugar + ½ cup cocoa.
Add ¼ cup heavy cream, 1 tsp vanilla, 1 tsp orange extract, zest of 1 orange, pinch of salt. Beat fluffy; adjust with more cream or sugar if needed.
Frost cupcakes. Garnish with orange zest or chocolate shavings.