Preheat the oven to 350°F. Line a regular 12-cup muffin tin with 12 cupcake liners. Set aside.

In a medium bowl, whisk the flour, cocoa powder, baking powder, and salt until it is well combined.

In a separate large bowl, rub the sugar and orange zest together until it is fragrant, then whisk in the oil. Add the eggs, vanilla extract, and 1 tablespoon of the orange juice (save the rest for the buttercream), and whisk until the eggs are just mixed. Use a spatula to fold in about half of the dry ingredients. Add the rest of the dry ingredients and pour in the milk. Stir until everything is just combined (a few small lumps in the batter is fine). The batter will be thick and wet.

Spoon the batter into the cupcake liners until they are about halfway full (about 2 tablespoons worth of batter), then bake the cupcakes for 17 minutes, or until a toothpick inserted in the center cupcakes comes out clean. Let the cupcakes completely cool.

After the cupcakes have cooled completely, use an electric mixer to combine the butter, cocoa powder, remaining orange juice, vanilla extract, and orange zest until smooth. Add the powdered sugar a little bit at a time and continue mixing until the buttercream is smooth and thick.

Transfer the buttercream to a piping bag, and pipe the frosting on the cupcakes. Garnish with orange sprinkles, Terry’s chocolate orange slices, or fresh orange slices.