Line 18 muffin tins with paper liners. Set aside for now.

Preheat oven to 350°F (180°C).

In a medium bowl, whisk together gluten-free flour, poppy seeds, baking powder, xanthan gum (if needed) and salt. Set aside for now.

In a large bowl, combine sugar, sour cream, melted butter, eggs, lemon juice, lemon zest and vanilla extract. Mix together well with a handheld mixer on low speed or mix by hand with a silicone spatula. Mix until fully combined.

Fill each paper-lined muffin tin until they are about ¾ full. There will be enough muffin batter for 18 muffins. Scooping Tip: This is a thick batter. To get perfectly round and even scoops into each paper liner, use an ice cream scoop (very lightly sprayed with non-stick baking oil) to scoop batter into each paper liner. Add coarse sugar on top if desired before baking.

Place muffins into the oven and bake for 22-24 minutes or until a toothpick inserted into the middle of the muffins comes out clean (without any wet raw batter left on the toothpick).

Let muffins cool completely before removing from muffin tins.

Glaze: Combine the powdered sugar and lemon juice in a small bowl and stir until smooth. Drizzle over cooled muffins.