Spray a 9x9 square baking pan and line with parchment paper if you want to be able to lift it out for cutting. This is optional.
Use a serrated vegetable peeler to carefully peel away the zest from the lemon. You want only the yellow, not any of the bitter white pith.
Put the sugar into the bowl of a food processor, along with the zest. Process, scraping down the sides if necessary, for about a minute or so, until the zest is completely incorporated into the sugar.
Add the flour, baking powder, and salt into the processor and pulse to combine everything.
Add the butter to the bowl. Pulse to combine, maybe about 20 pulses. The mixture should be crumbly with the butter well distributed and in very small pieces.
Measure out 1 cup of the mixture and set aside.
Whisk together the wet ingredients and mix with the dry, just until combined. Spread the batter into the pan, and top with the reserved crumbs. Note: I use my fingertips to make some larger, chunkier crumbs to create a nice texture on the top. Just compress the dry mixture a little between your fingers to make any size crumb you like.
Bake for 40-45 minutes, until a toothpick comes out without wet batter clinging to it.
I like to use organic lemons when I know I'll be using the peel, and I wash them really well.
If you don't have a food processor you can do this by hand. Grate the lemon zest and mix with the sugar. I would use the zest of 2 lemons if you are doing it by hand.