• Preheat the oven to 425°F (220°C). Add paper liners to a 12-cup muffin pan.

Make the Muffin Batter

• Melt the butter over medium-low heat and let it cool until it is barely warm (about 90°F). Crack the eggs into a small bowl and set aside.
• Place the all-purpose flour, baking powder, baking soda, and salt in a bowl and whisk for 30 seconds to combine.
• Place the sugar in a large bowl. Zest the lemons directly over the sugar until you have about 2 tablespoons. Use your fingers to rub the lemon zest into the sugar until it turns yellow and has a sandy texture.. Juice the lemons to yield 3 tablespoons of juice.
• Add the eggs and whisk them into the sugar for about 1 minute. Whisk in the cool melted butter. Next whisk in the sour cream, lemon juice, and vanilla extract. Let the batter sit for 5 minutes.
• Gently fold in half of the dry ingredients. Fold in the remaining flour mixture until no streaks of flour remain. The batter should be lumpy. Let the batter rest for 10 minutes.
• After the batter has rested, remove the blueberries from the freezer. Place them in a bowl and toss them with 1 tablespoon of flour until all the berries are coated. Add the berries and any flour in the bowl to the lemon muffin batter. Fold in the blueberries until they’re well distributed.

Bake the Muffins

• Divide the batter evenly among the 12 muffin cups (about 80 grams per muffin tin). The muffin cups should be almost full. Sprinkle about 1/2 teaspoon of sanding sugar over each muffin.
• Bake the muffins at 425°F (220°C) for 5 minutes, and then, without opening the oven door, reduce the temperature to 350°F (180°C).. Bake for another 20-22 minutes, until an instant-read thermometer registers about 200°F.
• Cool the muffins in the pan for 5 minutes, then move the muffins to a wire cooling rack.

Make the Lemon Glaze (Optional)

• Sift the powdered sugar into a small bowl. Add 3 teaspoons of lemon juice and stir. The glaze will be very thick. Add the remaining lemon 1/2 teaspoon at a time, stirring after each addition, until the desired thickness is reached.
• Drizzle the lemon glaze over the cooled muffins.

Store the Muffins

• The muffins can be stored at room temperature in an airtight container for up to 1 day. Store them in an airtight container in the refrigerator for up to 5 days
• To freeze the muffins, wrap each one in plastic wrap. Store the muffins in a freezer-safe bag or container. Label and date the bag and freeze for up to 2 months.
• To serve, thaw the muffins overnight in the refrigerator or for about an hour at room temperature. Warm the muffins in the microwave or in the oven for 5 minutes at 350°F (180°C).