Chocolate Cake with Potatoes

A wartime-economy cake recorded in shorthand: the ingredient list is complete but the method stops after the first step. The gaps are predictable, and the potato is the point.

• "2-3 cup butter"This is almost certainly two-thirds of a cup, about 150 g, the fraction having lost its slash somewhere between manuscript and page. Two to three cups of butter against two cups of flour would not be a cake.
• The unfinished methodThe likeliest completion: cream the butter and sugar with the yolks, work in the hot mashed potato and the melted chocolate, then add the flour sifted with the baking powder alternately with the milk. Since the surviving instruction mentions only yolks while the list calls for four whole eggs, the whites were probably beaten and folded in last, though the recipe breaks off before saying so.
• Hot mashed potatoThe recipe says only "hot mashed potatoes." Plain boiled potato mashed smooth, without added butter, milk or salt, keeps the fat and liquid in the recipe under control. Going in hot helps it blend rather than leaving lumps of cold starch in the batter. One cup is roughly one large potato, about 8 oz cooked.
• "2 rounding spoons baking powder"The size of spoon is not given. Two rounding tablespoons would be a very great deal of leavening for two cups of flour; two rounding teaspoons is closer to ordinary practice for a cake this size, and is the safer reading.
• Chocolate squaresA square meant one ounce of unsweetened baking chocolate, so two squares is 2 oz. Semisweet or chips will throw the sugar balance and the crumb.
• "Sweet milk"Ordinary fresh milk, specified to distinguish it from the sour milk or buttermilk that period recipes pair with soda.
• Pan and ovenNo baking direction is given at all. This quantity suits two 9-inch layer pans or one 9x13; 350F for 30 to 35 minutes for layers is a reasonable modern setting for a butter cake of this size.
• Keeping qualityThe potato starch holds water, so the cake stays moist for several days — the whole reason for the trick. Wrap it well rather than refrigerating, which dries it.