Preheat the oven and prepare the cupcake liners.

In a bowl, mix together the flour, baking powder, baking soda, and salt.

In a separate bowl, cream together the softened butter and granulated sugar.

Add the eggs, buttermilk, lemon juice, and lemon zest to the butter mixture and mix until combined.

Gradually add the dry ingredient mixture to the wet ingredients and mix until just combined.

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

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

Add the lemon juice, lemon zest, and milk and beat until smooth and creamy.

Once the cupcakes are cooled, frost them with the lemon buttercream.