Melt butter in soup kettle. Sauté cucumbers and celery until tender. Sprinkle with flour and stir until blended. Gradually add stock, stirring constantly, until mixture is smooth. Scald milk, onion, and mace in separate pan. Add milk mixture to kettle, and heat just to a boil. Combine cream and egg yolks in a bowl. Stir 1/2 cup of hot soup into egg and cream mixture and return mixture to kettle. Season to taste with salt and pepper. Cool and chill. Serve cold, garnished with chopped chives.