1. Preheat oven to 180 deg C.
2. Lightly grease a loaf tin with some butter then line the tin with some backing parchment. This will make it much easier to remove.
3. Using a spoon, beat the sugar and softened butter together well in a bowl. Beat in the eggs, egg white, and Greek yogurt. The mixture will start to look slightly curdled but keep mixing then add the flour, baking powder, vanilla or seeds and lemon zest. Stir until combined then spoon into the tin.
4. Bake in the centre of the oven until browned and a skewer inserted comes out dry.
5. Once baked remove the cake from the oven leaving it in the tin for a few minutes. Use the baking parchment to pull the cake from the tin and leave to cool completely on a wire rack.
6. Meanwhile make the glaze by mixing the lemon juice and sugar in a bowl until smooth and runny.
7. Once the cake has completely cooled place it on a serving plate and drizzle the glaze over it.