1. Preheat oven to 350ºF. Line a standard 12-cup muffin tin with paper or foil liners.
2. Make cupcakes: In a bowl, whisk flour, baking powder, baking soda and salt. In a large mixing bowl, using an electric mixer on medium-high speed, cream butter, sugar and orange zest until light and fluffy, about 3 minutes. Add eggs one at a time, beating well. Scrape down sides of bowl. Beat in half of flour mixture, until just combined. Beat in buttermilk and orange juice, then beat in remaining flour mixture until just combined (do not overmix).
3. Divide batter evenly among muffin cups. Bake until light golden brown and a toothpick inserted in center of a cupcake comes out clean, 20 to 25 minutes. Cool cupcakes in pan on a wire rack for 10 minutes, then remove from pan to rack to cool completely.
4. Make frosting: Beat butter, vanilla and milk together with an electric mixer until well combined and fluffy, about 2 minutes. Gradually add confectioners' sugar and beat on medium speed until combined. Raise speed to high and beat until fluffy and easy to spread, about 5 minutes. Frost cupcakes and serve.