With custard sauce. Compare the (custard-free) blancmange from Avon Lake, Ohio.
The recipe here isn’t much different from the one that appeared in the April 15, 1871 edition of The Janesville (Wisconsin) Gazette:
Half a box of gelatin, half a pint of cold water, let it soak half an hour, then add half a pint of boiling water. When cool, add the whites of there eggs, two cups of sugar juice of one lemon, and beat thoroughly. put in a mold to cool; make a boiled custard of the yolks, one and a half pints of milk–sugar to taste.
This does note an important point that the original card omits: that snow pudding would traditionally be served cold, so you ought to chill the pudding before serving.
From the box of J.L. from Westborough, Massachusetts.
3 egg whites
1 Tbsp. gelatin
1/4 cup cold H20
1 cup sugar
1/4 cup lemon juice
1 cup boiling H20
Dissolve gelatin in 1/4 cup cold H20, pour 1 cup boiling in. Add sugar and lemon juice. Let thicken to consistency of egg whites. Beat until foamy and add stiff egg whites. Serve with thick custard sauce.
2 cups milk
3 egg yolks
1/4 cup sugar
1/8 teaspoon salt
1 teaspoon vanilla
Scald milk in double boiler. Beat egg yolks till thick, adding sugar and salt. Pour half of scalded milk over beaten yolks and mix well. Return to double boiler and cook over hot H20 until mixture coats spoon. Flavor with vanilla.
Also, same recipe in Art of Pennsylvania Dutch Cooking by Edna Heller.