Fatty Cookbook: Swiss Meringue Buttercream

This was my first foray in to meringue. In high school, my French professor made these cookies, meringue obliées, which were whipped egg whites and sugar left to set in a cool oven. They were absolutely delicious, but I was terrified to ever make them myself. Desserts, and especially baking cookies, has long been my weak point in the kitchen. My cookies are always burnt on the bottom, and usually they spread and get stuck together and are too thin. I have one chocolate chip cookie recipe that I have mastered, but that's really as far as I've come.
We've got something like 8lbs of apples and peaches in the kitchen right now, and after dinner tonight I decided I had to have something sweet. Nothing like baked fruit with butter and brown sugar to satisfy that sweet tooth, and also to trigger some kind of diabetic coma. Not content with anything so simple, I went ahead and cracked open my Martha Stewart's Cooking School

From Fatty Cookbook

Swiss Meringue Buttercream
based on Martha Stewart original recipe

2 large eggs, separated
Pinch of salt
2/3 cup of granulated sugar
1/2 teaspoon cornstarch
8 tablespoons butter, softened

Set a small saucepan with 2" of water on the stove top to simmer. In a glass mixing bowl, combine egg whites, sugar, and salt. Let this rest while the egg whites reach room temperature. (In the meantime, I peeled some apples and cut up some peaches and set them in a pie tin to bake in the oven. This is accomplished by turning the oven on to any temperature and leaving the fruit in there for any amount of time.)
Place bowl over the saucepan to heat the egg whites and sugar, helping them to melt/mix easier. Whisk until sugar is dissolved and mixture is smooth.
The original recipe called for cream of tartar, but I don't have that and have never used it for anything. Add cornstarch instead, of the tartar if you have it and know what it's for. Now whisk the hell out of this stuff. Until it gets stiff and forms peaks. Most of you might do this by leaving the stuff to be whisked by your standing mixer on medium. I, you will remember, do not have a standing mixer. This is why my triceps are better than yours.

From Fatty Cookbook

Add the butter 2tbsp at a time, and whisk thoroughly in between. The buttercream mixture might look a little lumpy, or even curdled, but that will smooth out in the next step. The important part here is just to keep mixing. From the time the mixing bowl left the simmering water to the time the butter is fully mixed in took me about 15 minutes.

From Fatty Cookbook

You will see there that I also included chocolate chips. If the chocolate was melted, it would have blended right in and created chocolate buttercream. I, however, prefer my chocolate in bite sizes, so I let them mix in as is.
Now smooth out the whole mixture. Using a wide rubber spatula, turn the mixture several times to coax out air bubbles and ensure an even consistency. By hand or with the mixer, this should take 2-3 minutes.
Cover with plastic wrap and let sit for 5 minutes. The cream will thicken a bit, but will still be somewhat runny. Leave in the refrigerator over night for stiffer cream.