In a medium mixing bowl, combine the sugar and grated lemon zest. Using your fingertips, thoroughly rub the zest into the sugar until the mixture feels fragrant, clumpy, and pale yellow. This extracts the flavorful lemon oils and disperses them evenly, ensuring every muffin bursts with citrus.
To the lemon sugar, whisk in the sifted flour, baking powder, baking soda, salt, and poppy seeds. Mixing the dry ingredients together now guarantees all leaveners are evenly distributed, helping your muffins rise tall without scattered pockets of bitterness.
In a separate large bowl, whisk together the buttermilk, neutral oil, slightly cooled melted butter, eggs, lemon juice, and vanilla extract until completely smooth and homogenous. Use room temperature ingredients to prevent the batter from seizing and to make emulsification seamless.
Add the dry ingredient mixture to the wet, folding with a rubber spatula just until no flour pockets remain. Stop once you see only a few faint streaks; overmixing at this stage can develop excess gluten, making your muffins tough rather than tender.
Cover the bowl loosely and let the batter rest at room temperature for 15 to 18 minutes. This allows the flour to hydrate and the baking powder to begin reacting, building structure for signature domed tops. Meanwhile, preheat your oven to 425°F (218°C) and line a 12-cup muffin pan with paper liners.
Use an ice cream scoop or large spoon to divide the rested batter evenly among the 12 liners, filling each nearly to the top. For extra crunch and shine, sprinkle the surface of each with a light pinch of extra sugar. Give the pan a gentle tap to level the batter if needed.
Place the muffin pan in the preheated oven and bake at 425°F (218°C) for exactly 6 minutes to set the domes. Without opening the door, reduce the oven temperature to 350°F (177°C) and continue baking for 12 to 14 minutes. The muffins are done when the tops are deeply golden and a toothpick inserted in the center comes out with only a few moist crumbs, not wet batter.
Remove the pan from the oven and set on a rack for 5 minutes. Carefully lift each muffin out using tongs or your hands (protecting from heat) and transfer to the rack to cool completely. This prevents the bottoms from steaming and ensures the crumb sets to soft, plush perfection. Enjoy slightly warm or at room temperature.