Darwin's Potato Salad

Darwin makes this for us when he comes to visit and was generous enough to share the recipe.  This is from an old Amish Mennonite cookbook.

Salad:
8 medium potatoes
4 hard boiled eggs, chopped
1 medium chopped onion
1 cup chopped celery
8 radishes, chopped finely

Boil potatoes with skin on until soft.  Peel and dice into chunks.  Combine with hard boiled eggs, onion, radishes, and celery.

Dressing:
2 eggs
1/2 cup vinegar
1 tablespoon prepared mustard (yellow or dijon)
1/2 cup water
2 tablespoons butter 
1/2 cup sugar
1-1/2 teaspoons flour
1-1/2 teaspoons salt 

Combine eggs, vinegar, mustard and water in a saucepan.  Heat mixture on medium heat, then add butter, sugar, flour and salt.  Cook until thickened, stirring frequently.  Remove from heat, and let it cool for a few minutes.  Pour over potato mixture, combine and refrigerate for a few hours before serving.

Optional additions: 2 small carrots, 1/2 cup mayonnaise if desired.