Preheat oven to 350 degrees F.
Cream together butter and sugar.
Beat in eggs one at a time, then stir in vanilla.
Combine flour, baking powder, and salt; gradually add to the wet ingredients.
Stir in lemon juice and lemon zest.
Pour batter into a greased 8x8 inch pan.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
