Unsalted butter tastes a little sweeter than the more common salted butter. We added the coarse salt for small bursts of saltiness to complement the sweetness of the other ingredients and to bring out the lemon flavor. If you don't have unsalted butter, salted butter can be used and omit the kosher salt.
If you have only one pan and a recipe calls for more cupcakes than your pan will make, cover and refrigerate the rest of the batter while baking the first batch. Cool the pan about 15 minutes, then bake the rest of the batter, adding 1 to 2 minutes to the bake time.