Put the water in a saucepan with a pinch of salt and sugar and bring to the boil. Add asparagus and simmer 15 minutes until soft. Strain and retain the cooking liquid.
Melt 40g butter in the saucepan, add flour and saute, stirring. Gradually stir in cooking water so no lumps form. Puree the asparagus and add to the soup. Add chicken stock granules and bring to a simmer.
Beat the egg yolk and cream together. Reduce heat and carefully stir in the cream mixture - do not let it boil or it may curdle. Add remaining 75g butter and let melt. Season with salt and white pepper.