Preheat oven to 350°F. Line a 12-count muffin pan with liners or lightly grease the muffin cups.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and poppy seeds. Set aside.
In a stand mixer or large mixing bowl, cream the room-temperature butter and sugar together until light and fluffy.
Add the egg and mix until combined. Add the sourdough discard, ricotta cheese, milk, lemon zest, and vanilla extract. Mix until smooth.
Add the wet ingredients to the dry ingredients and gently fold together with a spatula until just combined. Be careful not to overmix. The batter will be thick.
Allow the batter to rest for 30 minutes before baking, or cover and refrigerate overnight for a longer sourdough fermentation.
Divide the batter evenly between the 12 muffin cups. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean.
Transfer muffins to a cooling rack and allow them to cool completely. To make the glaze, sift powdered sugar into a bowl, then add lemon juice and melted butter. Stir until smooth. Drizzle over cooled muffins before serving.
