In a medium bowl, use a fork to mix the sugar with the lemon zest. This will release the oils in the zest and help get your cupcakes full of lemon flavor!
Add the flour, sugar, salt, baking power and baking soda to the sugar mixture and whisk together.
In another bowl whisk the sour cream, lemon juice, vegetable oil, egg and vanilla together. Add the dry ingredients to the wet and gently mix until just combined.
Scoop evenly into prepared muffin pan. Bake for 16-18 minutes, until the top of the cupcakes spring back when lightly pressed or a toothpick comes out clean. Cool in the muffin pan for 10 minutes or so and then remove cupcakes from the pan and allow to cool completely.
Lemon Cream Cheese Frosting
In the bowl of a mixer, beat the butter and cream cheese together with the paddle attachment until combined and smooth, about 2 minutes.
Add the vanilla, salt, lemon juice and lemon zest and beat again for 1 minute.
Add 4 ½ cups of powdered sugar and mix on low until the sugar is mostly absorbed by the wet ingredients. Scrape down the sides and bottom of the bowl and then beat again on medium high until smooth and fluffy. If the frosting is not as stiff as you would like it, add the remaining cup of powdered sugar and beat again until smooth and stiff.