zest of 1 lemon, peeled with a vegetable peeler into strips. You want the yellow part, with as little of the bitter white as possible. Note: you don't want the peel of the lemon, just the outer zest. Use a serrated vegetable peeler for best results and just peel off the yellow part, leaving the bitter white behind.
Preheat oven to 350F. Lightly spray and line a 9x9 baking dish with parchment paper. The paper is optional but helps lift the cake out for easy slicing.
Put the sugar and zest in a food processor and process until the zest is completely incorporated, and the sugar is moist and pale yellow. Note: if you don't have a processor then grate the zest of 1 lemon and mix with sugar by hand.
Cream the butter and sugar together in a stand mixer, or in a bowl with a hand held mixer.
Beat in the eggs, scraping down the sides of the bowl as necessary.
Whisk together the flour, baking powder, baking soda, and salt in a separate bowl.
Add the lemon juice and extract to the buttermilk.
Alternately add the flour and buttermilk to the creamed sugar and eggs. Begin and end with the dry ingredients. Blend until everything is well combined, but don't over mix.
Spoon the batter into the prepared pan and smooth out evenly. Sprinkle the sugar evenly over the entire surface. Note: I know, it seems like a lot, but go with it, it makes a nice crust.
Bake on the middle rack for about 30 minutes, or until golden.