1. Preheat the oven to 350°F and grease two 8-inch round cake pans. Line the bottoms with parchment paper.

2. In a medium bowl, whisk together the flour, baking powder, and salt.

3. In a large bowl, beat the butter and granulated sugar until pale and fluffy.

4. Add the eggs one at a time, mixing well after each addition.

5. Mix in the lemon zest, lemon juice, and vanilla extract.

6. Add the dry ingredients in three additions, alternating with the milk and sour cream. Mix just until combined.

7. Divide the batter evenly between the prepared pans and smooth the tops.

8. Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out clean.

9. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

10. For the frosting, beat the cream cheese and butter until smooth.

11. Gradually add the powdered sugar, then mix in the lemon juice and lemon zest.

12. Add heavy cream, 1 tablespoon at a time, until the frosting is smooth and spreadable.

13. Place one cake layer on a serving plate and spread frosting over the top.

14. Add the second layer and frost the top and sides of the cake.

15. Chill briefly for cleaner slices, then serve.