Preheat oven to 375F. Line the cupcake tin with papers.
Add the sugar and lemon zest to the bowl of a food processor then pulse until zest is broken up and the sugar is pale yellow In color.
Sift the flour, baking soda, baking powder and salt into a large bowl then whisk together and set aside.
Add the eggs, milk, melted butter, lemon juice, sour cream, and sugar to a medium bowl and whisk together. Everything should be room temperature, but If things clump up after mixing then just microwave the bowl for 30 seconds and mix once more.
Add the wet to the dry mixture and mix until just combined. Fill cupcake papers about 3/4 the way up. Bake for 5 min at 375F then reduce temperature to 350F and bake another 10-12 or until the springy to the touch.
For the Buttercream:
Cream the butter, salt and zest for a few minutes then sift in the powdered sugar in a few batches, mixing in between.
Scrape the bowl down and pour In the lemon juice while mixing on low. Scrape down once more and mix then transfer to a piping bag and decorate the cooled cupcakes.
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Notes
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best and easiest way to measure flour is by using a scale. If you don’t have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
If your frosting is runny then add more powdered sugar a cup at a time until the consistency thickens up.
Give your frosting a mix before using for best results.
NEVER use bottled lemon juice! It really will not taste the same.
You can substitute whole milk yogurt for the sour cream if desired, I do it all the time and can’t really tell the difference.
If you're using lemon zest for garnish then add just before serving. It will dry out fairly quickly.