Position a rack in the lower third of the oven. Preheat the oven to 350°F. Butter and flour a 10 × 2-inch round cake pan or 9-inch square baking dish.
Prepare the streusel topping...
Of all the types of recipes out there, baking recipes can be the most cryptic.
After all, how often do you see phrases like, "Have ready 1/4 cup mirepoix in brunoise" in your standard cookbook. Or perhaps, "Have ready one chicken en crapaudine..." It's just not gonna happen. For better or for worse, these days, you have to clearly enunciate through the abbreviated format of the recipe. Otherwise, your recipes risk being dismissed outright by the harried, simplicity-seeking cooks of today.
However, baking recipes still use an erudite shorthand that some new bakers find intimidating. How do you temper chocolate? How do you cream butter and sugar? What the heck are stiff peaks? As a baker, I find these phrases reassuring. An age-old collection of jargon that tells me exactly what to do. But to many, these instructions are enigmatic and need decoding before they can be useful or helpful.
A long time ago, I posted instructions on separating eggs whites and yolks. You'll want to reference that before beginning the next exercise: whipping and folding egg whites into batter.
Whipping egg whites and folding them into batter is often performed when making chiffon or génoise cakes or when lightening up a normal cake recipe. Some pound cake recipes call for you to do this in order to make them lighter.
Before you whip egg whites, wash your mixing bowl with warm, soapy water. You want to be sure that there are no oil or grease particles in your bowl. Even if you haven't used your mixing bowl recently, I recommend washing it. In the kitchen, grease tends to build up almost everywhere, as I recently found out while washing our windowsills.
Dry the bowl thoroughly. Do the same for your whisk. Place your egg whites along with the amount of cream of tartar specified in the recipe in the mixing bowl. Cream of tartar is a byproduct of wine making that is often used when whipping egg whites to stabilize the egg foam and increase the volume of the whites.
I like to whip my egg whites on medium speed (I use a KitchenAid stand mixer. For handheld mixers, use high speed). I find that they retain their structure better when whipped slower. Begin whipping and keep an eye out for them to foam and thicken.
Beat the egg whites until soft peaks form. You'll know you've reached this stage when you can lift up the whip, and the tip of the peak formed by lifting the whip droops over. At this point, gradually add the sugar called for in the recipe.
Continue whipping at medium speed until the egg whites form stiff, glossy peaks. You can see this clearly by lifting up your whip and holding it horizontally. The egg whites should not droop or fall off the whip.
It is important not to overbeat at this stage. If you whip until the egg whites are dry, the mixture will break down as you fold it into the batter, creating a heavier end product. I like to stop my mixer frequently as I get close to the stiff peak stage. This way, I can check the consistency of the whites and make sure I don't overbeat them.
As soon as the whites have reached this stage, immediately fold them into your batter. To do this, first add a third of the egg whites to the batter and fold them in gently. This is to lighten the batter initially and prepare it for the rest of the egg whites.
When I use the term "fold," I mean to use a rubber spatula. Gently plunge the spatula down (hold it so that the flat side is facing you--you should be able to see the expanse of the spatula's head) in the center of the mixture. Draw the spatula towards you, scooping a swath of batter along with it. Bring the spatula up the side of the bowl and plunge it down in the center of the batter again. You should see a ribbon of batter left in the wake of the spatula. As you repeat this movement, rotate the bowl, working your way around, always starting in the center, moving to the side of the bowl, and bringing the spatula upwards to mingle the egg whites and batter. As you fold, the position of the spatula will change. When you plunge it down in the center, you want the flat side towards you, meaning that the "sharp" side goes down into the batter first, causing as little deflation of the whites as possible. When you bring the spatula up again, you want the flat side facing up in order to bring up as much batter as possible.
Repeat this movement until the batter is more or less homogenous. It doesn't have to be perfect. You don't want to see large streaks or clumps of egg white, but total and complete incorporation of egg whites is not necessary. You may see some thin, fine stripes of egg white. This is okay.
Immediately transfer the batter to your pan and finish the recipe as instructed.
Once you find your rhythm, this technique is actually very satisfying. It also has a way of making you look very accomplished and professional, so make sure someone is watching you as you do it. It's not very often that we get to feel accomplished and professional.