Mix a and heat just to boiling. Slurry b together and pour into a with constant stirring. Stir until thick and smooth; bring back to boil. Dry-mix c by sifting together several times. Add slowly to a-b with constant stirring. Stir until smooth and thick. Mix egg yolks with a portion of preceding mix, then stir into entire mix and cook on steam heat for about 2 minutes. Stir in butter; then stir in lemon juice.