Place a couple of inches of water in the bottom of a small pot or double boiler. In a bowl, mix lemon juice, zest, sugar, egg yolks, egg, and salt. Whisk together.

Place the bowl over the steaming water and stir continually until the mixture thickens.

Set a fine mesh sieve over a bowl, pour the lemon curd into it, and stir to encourage it through. Stir in butter until melted. Cover with plastic wrap and chill.

In a mixing bowl, beat together the softened butter and sugar. Stir in lemon zest, then add sour cream and lemon juice, followed by the eggs until a smooth mixture forms.

In a separate bowl, whisk flour, baking powder, and salt. Fold the dry ingredients into the wet ingredients without overmixing.

Preheat oven to 400°F. Line a muffin tin with paper liners. Fill each cup halfway with batter, hollow out a bit in the center, add lemon curd, and cover with more batter.

Mix granulated sugar with lemon zest and sprinkle over each muffin. Bake for 18-20 minutes until lightly golden around edges.

Sift powdered sugar into a bowl, add lemon juice and stir to make a glaze. Once muffins cool slightly, drizzle glaze over with a spoon.