Preheat oven to 350F
Whisk together egg yolk and sugar until mixture looks pale yellow and bubbly
Whisk in melted butter, sour cream and lemon juice
Sift in dry ingredients (flour, baking powder, salt) and gently fold until just combined
Bake for 18-20 minutes until golden
Mix powdered sugar, lemon juice, and vanilla to make icing
Apply lemon glaze to cooled loaf
