Preheat your oven to 375F. Line a 12-cup muffin tin with paper liners, or grease well with butter or cooking spray.

In a large bowl, whisk together the flour, sugar, poppy seeds, baking powder, baking soda, and salt until well combined. Break up any clumps of poppy seeds so they distribute evenly.

In a medium bowl, whisk the Greek yogurt, melted butter, eggs, lemon zest, lemon juice, and vanilla until smooth. The mixture will look slightly curdled from the lemon - this is normal.

Pour the wet ingredients into the dry ingredients. Stir with a rubber spatula until just combined - about 10 to 12 strokes. The batter should be thick and slightly lumpy; do not overmix or your muffins will be tough.

Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. Bake for 16 to 18 minutes, until the tops are golden and a toothpick inserted in the center comes out clean or with just a few moist crumbs.

Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before glazing.

Whisk the powdered sugar and lemon juice together until smooth and pourable, adding more juice by the teaspoon if needed. Drizzle over the cooled muffins and let set for 15 minutes before serving.