Patio Potato Salad... in A Day in the Life

I found this recipe in a church cookbook.

Dressing :
1/2 cup milk
1/3 cup sugar
1 egg
1/4 cup vinegar
4 tablespoons margarine
1 tablespoon cornstarch
3/4 teaspoon salt
3/4 teaspoon celery seed
1/4 teaspoon dry mustard

Combine dressing ingredients in a medium saucepan. Cook over low heat, stirring constantly until thick. Remove from heat. Cool.
1/4 cup onion, chopped
1/4 cup mayonnaise
7 medium potatoes, cooked and diced
3 eggs, hard-boiled and chopped

Combine onion, mayonnaise, potatoes and eggs. Fold in dressing and chill. Before serving, sprinkle with paprika.

I wonder if I could substitute butter for the margarine? I like to use butter when I bake and cook.