Preheat oven to 375°F (190°C) and line a muffin tin with paper liners.
Whisk flour, sugar, poppy seeds, baking powder, baking soda, salt, and lemon zest in a bowl.
In another bowl, whisk melted butter, eggs, buttermilk, lemon juice, and vanilla.
Pour wet ingredients into dry ingredients and mix just until combined.
Divide batter evenly into muffin cups, filling about 3/4 full.
Bake 16–20 minutes until tops are golden and a toothpick comes out clean.
Cool slightly before adding glaze if desired.
Mix glaze ingredients and drizzle over warm muffins.