In a saucepan over medium heat, whisk together sugar and egg until smooth. Whisk in melted butter, lemon juice, and lemon zest. Keep whisking mixture until it thickens, approximately 15 minutes. (It will thicken more as it cools.) Let glaze cool for 20 to 25 minutes before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/ellery-adams-serves-suspense-and-scones-in-the-vanishing-type/