• Mix the all-purpose flour, baking powder, baking soda, and salt in a medium bowl. Set aside. 250 g all-purpose flour, 2 teaspoons baking powder, 1/4 teaspoon baking soda, ½ teaspoon salt
• In a large bowl, add granulated sugar and lemon zest. Use your fingertips to gently rub the zest into the sugar to release the lemon oils. 200 g granulated sugar, 2 Tablespoons lemon zest
• Add the melted butter, oil, lemon juice, eggs, sourdough discard, and vanilla extract. Whisk until well combined. 113 g butter, 15 g vegetable oil, 30 g lemon juice, 2 large eggs, 1 teaspoon vanilla extract, 100 g sourdough discard
• Add the flour mixture to the wet ingredients and the milk and fold with a rubber spatula or wooden spoon until combined. 180 g milk
• The muffins can be baked right away or cold-fermented overnight.
• Cold-fermenting (optional): Cover the bowl tightly and place it in the refrigerator for up to 16 hours. Cold fermenting allows the flour to absorb more moisture from the batter, which can lead to a more moist crumb. Additionally, slow fermentation helps break down the proteins in the flour. The next day the batter will be very thick.

Baking

• Preheat the oven to 425°F (220°C) and line a muffin pan.
• Use an ice cream scoop to divide the muffin batter evenly over the 12 liners. They will be full.
• Bake for 5 minutes in the preheated oven, then turn the oven down to 350°F (180°C) and bake for 16-18 minutes more until golden brown and the muffin tops spring back when pressed or a cake tester inserted in the middle comes out clean. 
• Let the baked muffins cool in the pan for 5 minutes before removing them and placing them on a wire rack to cool further.

Lemon glaze

• Combine the powdered sugar and lemon juice in a small bowl. If you'd like an extra hit of citrus flavor, you can also add some lemon zest. Stir everything together until you have a smooth, slightly runny glaze. 90 g powdered sugar, 1 1/2 TBSP lemon juice
• If the glaze is too thick, add a bit more lemon juice. If it's too thin, add more powdered sugar.
• Once the muffins have cooled down a bit but are still slightly warm, drizzle the glaze over the muffins.