In a mixing bowl, combine sugar, lemon zest, flour, baking soda, baking powder, sea salt and poppy seeds.
In a separate bowl, mix and combine eggs, kefir and vanilla extract. Add cooled, melted butter. Mix and combine.
Add lemon juice and stir to combine all liquids.
Add egg mixture to dry ingredients and stir just until moistened. Batter should be lumpy!
Spoon batter into 12 muffin tin cups lined with paper liners. Bake in the oven for 12-16 minutes at 200°C until an inserted toothpick comes out clean. Cool muffins on a wire rack about 5 minutes, then drizzle the glaze over each muffin. Remove from muffins pan and slip off paper liners.
Lemon glaze: combine sifted powdered sugar and lemon juice; mix, adding additional lemon juice or sugar, until consistency of glaze can be drizzled from a spoon.