Cupcakes: Preheat oven to 350° F.

In a large saucepan melt the butter with the lemon juice and water on medium high heat. When the butter is melted remove from heat, add sugar and stir until dissolved.

Whisk in the sour cream (or yogurt), beaten eggs, vanilla extract and lemon zest, and let the mixture cool completely before preceding. In a separate bowl whisk the flour, salt and baking soda together, then whisk the dry ingredients into the wet to make a batter that is somewhat thin.

Prepare muffin pans with cupcake liners. Pour or ladl) the batter into the cupcake liners 3/4 of the way full, this is important to the success of the cupcakes. Bake for 18-20 minutes, until a toothpick inserted into the middle of a cupcake comes out clean. Don't open the oven until 18 minutes have passed. Cool cupcakes on a wire rack for 10 minutes. Remove cupcakes from pan and let cool completely before frosting.

Frosting: Beat the butter well, about one minute if using a stand mixer or two minutes with a hand mixer. Add about half of the powdered sugar, the vanilla, lemon zest and juice, then beat well starting very slowly. Once it is incorporated add the rest of the sugar, then increase to high speed and beat until very fluffy, 3 to 5 minutes.