Preheat oven to 425F (220C) and line a 12-count muffin tin with paper liners (or make your own parchment paper liners).
In a large bowl, whisk together flour, sugar, baking powder, baking soda, cornstarch, poppy seeds, and salt.
In a separate bowl, whisk together sour cream, melted butter, egg, lemon zest, lemon juice, vanilla extract, and lemon extract (if using).
Add the wet ingredients to dry ingredients and use a spatula to gently stir together until just combined (don’t over-mix or your muffins will be dense and dry). The muffin batter will be very thick, this is normal!
Evenly divide batter into prepared muffin tin (for tall muffins with big muffin tops like in the photos, only divide into 10 or 11 cavities rather than all 12) and transfer to 425F (220C) preheated oven. Bake for 8 minutes and then, without opening the oven door or removing the muffins, reduce heat to 350F (175C) and bake another 5-6 minutes if baking twelve muffins and 7-8 minutes if baking ten muffins, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Allow to cool before drizzling with glaze.
To prepare glaze, whisk together powdered sugar, lemon juice, and water until proper consistency is reached (not too runny, stiff but thin enough to drizzle). Drizzle over cooled muffins. Allow glaze to harden a bit before serving.
¹When zesting your lemon, don’t zest too deep into the white papery “pith” or your muffins will be bitter.