Preheat the oven and prepare a cupcake pan.

In a bowl, cream together unsalted butter and granulated sugar until light and fluffy.

Add eggs one at a time, beating well after each addition.

Mix in buttermilk, lemon juice, and lemon zest until well combined.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing just until incorporated.

Fill cupcake liners in the pan and bake for 20 minutes or until a toothpick comes out clean.

For the frosting, beat softened butter and powdered sugar together.

Add lemon juice and lemon zest to the frosting, mixing until smooth.

Adjust consistency with milk if necessary, then frost cooled cupcakes.