Prep your oven and pan. Position a rack in the center of the oven and preheat the oven to 400°F. Prepare a muffin pan by spraying 4 cavities with cooking spray. If you're using a standard muffin pan, position the pan so that its short side faces you. Then, prep the pan by spraying down one column of the cavities with cooking spray. Pour water into the remaining cavities, filling them at least ⅔-rds of the way full.
Prep the ingredients. In a medium bowl, whisk together the flour, baking powder, and salt. In a small bowl, combine the sugar and lemon zest. Use your fingers to rub the zest into the sugar—this will infuse the sugar with oils from the zest. In a small liquid measuring cup, whisk together the buttermilk, lemon juice, and lemon oil.
Make the muffin batter. In the bowl of a stand mixer fitted with the paddle attachment, combine the sugar, lemon zest, and butter. Beat on medium-high until light, fluffy, and doubled in volume, 2 to 3 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Reduce the mixer to low and add the egg.
With the mixer on low, add the dry ingredients in two equal parts, adding the second half only when the first half has been just combined. Beat until just combined, then scrape down the bottom and sides of the bowl once more, then beat on low for an additional 30 seconds. Slowly pour in the buttermilk mixture in a steady stream down the side of the bowl and beat until just combined, 30 to 45 seconds. Scrape down the bottom and sides of the bowl once more, then add the poppy seeds all at once. Beat on low until just combined.
Assemble the muffins. Use a 1-Tablespoon cookie dough scoop to fill each prepped cavity with 5 Tablespoons of batter. Sprinkle the top of each muffin, aiming for the batter and avoiding the pan, with 1 teaspoon of granulated sugar. Garnish with poppy seeds. Bake the muffins. Bake for 22 to 24 minutes, or until the muffins are domed and golden brown around the edges. A skewer inserted into the center of a muffin should come out with few crumbs attached. Cool the muffins in their muffin tin on a wire rack for 5 minutes, then turn the muffins out onto the rack. Turn out the muffins by gently pinching an individual muffin around the base of its muffin top and lifting it out of the pan.
Serve and store. Serve warm or at room temperature. The muffins can be wrapped individually in plastic wrap and stored at room temperature for up to 2 days.