Preheat oven to 375 degrees F. Line a 12-cup muffin time with paper liners.
Melt butter and set aside to cool.
In a large bowl, whisk together flour, sugar, poppy or chia seeds, baking powder, baking soda, and salt
In a medium bowl, whisk yogurt, lemon zest, vanilla, and eggs until smooth. Gently fold yogurt mixture into flour mixture until just combined. Fold in melted butter. Do not overmix. If you overmix, the muffins will be dense.
Divide batter evenly among prepared muffin cups. I like to use an ice cream scoop for this. Bake until tops are golden brown and a toothpick inserted in the center comes out clean, about 16-20 minutes. Rotate muffin tin halfway through baking.
While muffin are baking, combine confectioners sugar and lemon juice together in a small bowl until it forms a syrup.
Remove muffins from oven and brush immediately with lemon glaze. Sprinkle with coarse sugar, if desired. Let muffins cool in tin for 5 minutes, then transfer to a wire rack and let cool for another 10 minutes before serving.
Store in airtight container for up to 3 days.