Preheat your oven to 350 degrees F. Coat the inside of a 9-inch x 13-inch baking dish or pan with non-stick cooking spray. Set aside.
Lemon Cake
Whisk together the flour, baking powder, and salt in a medium mixing bowl. Set aside.
Add the butter and sugar to a large mixing bowl. Using a hand mixer on medium speed, beat until light and fluffy, 1-2 minutes. Add the eggs, one at a time, mixing well after each addition. Add the lemon juice and lemon zest. Mix to combine.
Add half of the flour mixture to the bowl and mix until just combined. Repeat with the rest of the flour. Stir in the sour cream. (see note)
Transfer the batter to the prepared baking dish.
Bake for 25-30 minutes, or until a pick inserted into the center comes out clean
Topping
Whisk together the sweetened condensed milk and 1/4 cup lemon juice in a medium bowl until smooth.
Use a fork or the back of a wooden spoon to poke holes over the entire surface of the warm cake. Pour the sweetened condensed milk mixture all over the cake. (Not all of the mixture will immediately be absorbed into the cake. That's okay.)
Allow the cake to cool completely on a wire rack. Once cooled, refrigerate the lemon cake for at least 1-2 hours to allow the topping to set and soak into the cake.
Cream Cheese Frosting
Beat together the cream cheese, powdered sugar, and 1 tablespoon of lemon juice until smooth. (see note)
Spread the frosting evenly over the top of the cooled cake. Refrigerate for at least 1 hour before serving.
Garnish the top of the cake with lemon zest and quartered lemon slices, if desired. Cut the cake into 12 slices, serve, and enjoy!
Notes
You'll need about 3 lemons total for this recipe. Most lemons give 2-3 tablespoons of juice. If you prefer to use bottled lemon juice, that's fine, but freshly squeezed tastes the best in this recipe.
If you want the cake to be yellow, you'll need to add some yellow food coloring to the batter before baking.
If the frosting is too stiff, add a little more lemon juice to thin it out. If the frosting is too thick, add more powdered sugar until it has thickened. I usually end up adding a couple of extra tablespoons of powered sugar.