Prepare: Preheat oven to 350°F (176°C) and line a 12 count muffin tin with paper liners, or lightly grease.
Dry ingredients: Whisk the flour, salt, baking powder, baking soda and poppy seeds together in a bowl and set aside.
Wet ingredients: Add the room temperature butter and sugar to a mixing bowl and use an electric hand mixer to cream the butter and sugar together. Add the egg, sourdough discard, milk, ricotta, lemon zest and vanilla extract and mix until smooth.
Combine: Add the wet ingredients to the dry ingredients and mix with the spatula until just combined making sure not to over-mix. The batter will be thick. After mixing, either allow the batter to rest for 30 minutes before baking or, place the batter in the fridge to ferment overnight. (See notes.)
Bake: Divide the batter evenly into the 12 liners. If the batter has been chilling in the fridge, it will be stiff. Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean. Transfer to cooling rack and cool completely before glazing.
Glaze: Mix the powdered sugar, lemon juice and butter in a small bowl until smooth. It will take a minute of stirring for the butter to incorporate. Drizzle over the muffins or dip the top of each muffin into the glaze.