Heat oven to 350°. Mix the lemon juice and milk and set aside. Grease and flour a loaf pan. In the bowl of a stand mixer, beat butter and sugar until light and fluffy. Beat in the egg, vanilla and lemon zest. In a separate bowl combine the flour, baking powder and salt. Add half to the butter mixture and mix until just incorporated. Add half the milk, then the second half of the flour mixture, ending with the milk, mixing just until incorporated after each addition.