Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Whisk flour, sugar, baking powder, baking soda, and salt in a large bowl.
In another bowl, cream butter until smooth. Beat in eggs one at a time.
Add lemon juice, lemon zest, and vanilla extract, mixing well.
Alternate adding flour mixture and buttermilk to butter mixture, starting and ending with flour. Mix until just combined.
Divide batter evenly into pans; bake 30–35 minutes, until a toothpick comes out clean.
Cool cakes in pans for 10 minutes, then transfer to wire racks to cool completely.
For frosting: Beat butter until creamy. Gradually add powdered sugar, then mix in lemon juice, zest, vanilla, and salt until fluffy.
Frost cooled cakes between layers, on top, and around sides.