Step 1: Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 4: Beat in the eggs one at a time, incorporating each egg fully before adding the next. Stir in the lemon zest.
Step 5: With the mixer on low speed, alternately add the dry ingredients and the buttermilk to the wet ingredients, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.
Step 6: Stir in the fresh lemon juice until just combined.
Step 7: Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Step 8: Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean.
Step 9: Remove the cupcakes from the oven and let them cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely.
Step 10: To make the frosting, in a large bowl, using an electric mixer, beat the softened butter on medium-high speed until creamy and pale, about 3-4 minutes.
Step 11: Gradually add the powdered sugar, one cup at a time, mixing on low speed until combined, then increasing to medium-high. Continue beating until light and fluffy.
Step 12: Beat in the fresh lemon juice and lemon zest until smooth and combined. If the frosting is too thick, add a tiny bit more lemon juice (1/2 teaspoon at a time) or a splash of milk until desired consistency is reached.
Step 13: Once the cupcakes are completely cool, frost them using an offset spatula or a piping bag with your favorite tip. Garnish with additional lemon zest, if desired.