Preheat the oven to 375°F
Melt the butter and let it cool
Whisk together the flour, sugar, poppyseeds, baking powder, baking soda, and salt
Whisk together the sourdough discard, yogurt, melted butter, vanilla, lemon juice, and lemon zest until smooth
Fold in the eggs
Add the wet ingredients to the dry ingredients and mix just until combined
Grease a 12 cup muffin pan
Fill each muffin cup about ⅔ full
Bake for 20 minutes or until a toothpick comes out clean
Let the muffins cool on a rack
Add the lemon juice and sugar to a small saucepan
Bring it to a boil over medium heat and simmer 3 to 5 minutes until slightly thickened
Brush the glaze over the muffins while they are still warm
Top with coarse sugar
