Perhaps the best way to illustrate how reduced-fat baking works is to tell the story of how I gave my mom's chocolate fudge cake a makeover with Fudgy Chocolate Frosting. It didn't happen overnight.
I have vivid memories of Mom in the kitchen, instinctively mixing up her famous chocolate fudge cake in a big bowl. She made it so many times, she never had to look at the recipe. I learned it by heart, too: two sticks of butter, two cups of sugar, two whole eggs (Mom preferred double-yolk), one-half cup of sour cream, two cups of sifted all-purpose flour, three ounces of unsweetened chocolate, one cup of hot brewed coffee, one teaspoon of baking soda, and a pinch of salt. The frosting had four more ounces of chocolate, one pound of confectioners' sugar, and one teaspoon of vanilla, mixed with a whole raw egg (I doubt if she'd use a raw egg today) and a few tablespoons of whole milk.
When my family decided to pay more attention to our diet, I knew that Mom's chocolate fudge cake, as it stood, didn't fit into the plan—especially after I ran the numbers on the original recipe. I almost fainted when the calculations showed 603 calories, 28 fat grams (11 grams of saturated fat), and 100 milligrams of cholesterol per slice (twelve slices per cake)! If I had announced to my gang that they would never eat chocolate cake again, they would have packed their bags and found a new mom and wife. But I knew that somewhere in Mom's beloved recipe there was a wonderful reduced-fat version just waiting to be discovered.
So, I rolled up my sleeves and got busy. Obviously, the place to start was with the butter. Fat serves several purposes in a baked good, primarily contributing to and enhancing the flavor. After much experimentation, I found that I could cut the fat down from sixteen to four tablespoons – 25 percent of its original amount—before the flavor and texture were adversely affected. (in other recipes, substituting a fruit puree like applesauce for part of the fat works well.) I also substituted low-gluten cake flour for the all-purpose flour, as cake flour will produce a more tender cake, which can be a problem win the absence of fat.
To make up for the flavor lost by reducing the amount of melted chocolate, I added one-half cup of cocoa powder, which is a surprisingly low-fat ingredient (see How Chocolate Works for details). In addition I added two teaspoons of instant espresso to complement the chocolate flavor. I used low-fat buttermilk which has a similar full, rich flavor to sour cream, but fewer calories and less fat.
Egg yolks provide fat and lecithin (a natural emulsifier), which contribute to the fine texture of baked goods, and egg whites contain proteins that give structure to the final product. I could have substituted four egg whites for the two whole eggs, but I kept one egg and substituted two whites for the other. The little bit of lecithin in that one yolk made a big difference. Too many egg whites will make a baked good dry and rubbery.
Finally, I added an important instruction to the recipe: "Do not open the oven until the last five minutes of baking." All low-fat and reduced-fat baked goods are extremely sensitive to shifts in oven temperature (which occur when the door is opened) and could fall.
Of course, I didn't get it right on the first try. But, get it right I did, because I came to understand the interplay between the ingredients and the techniques used in reduced-fat baked goods. And finally, after many trials, I came up with a rich, delicious, chocolate cake and a new fudgy frosting that my family loves. This Chocolate Fudge Layer Cake with the Fudgy Chocolate Frosting, was reduced by over 280 calories, 18 grams fat (6 grams saturated fat), and 69 mg cholesterol, per slice!