2Grease (or spray with non-stick cooking spray) three 8" or 8 1/2" round cake pans.
3In a small, heavy saucepan over lowest heat, melt chocolate, stirring constantly until smooth; be careful not to scorch.
5Sift together flour, cocoa powder, baking powder, baking soda and salt onto a sheet of wax paper.
6In a large mixer bowl, with mixer set on medium speed, beat oil, butter and sugar until very well blended and fluffy.
7Beat in chocolate.
8One at a time, beat in egg, then whites, coffee and vanilla until smoothly incorporated.
9Gently stir half of the dry ingredients, then yogurt into the mixture just until mixed.
10Stir in remaining dry ingredients just until well blended and smooth.
11Divide batter among pans, spreading in edges. Bake in the middle of the oven for 23-28 minutes, or until tops are almost firm when tapped and a toothpick inserted in the center comes out moist but clean.
12Transfer pans to racks and let stand until completely cooled.
13Layers may be wrapped airtight and frozen for later use.
14Let return to room temperature before using.
15Or frost cake and serve immediately, if desired.
16FLUFFY CHOCOLATE FROSTING: Place egg whites in a large mixer bowl.
17Set the bowl in a large bowl of very hot tap water and let stand for 10 minutes, stirring occasionally.
18In a small heavy saucepan set over low heat, melt chocolate, stirring constantly, until smooth; be very careful not to scorch.
19Set aside to cool slightly.
20Combine the corn syrup, 1/4 cup water and the sugar in a 2-quarter saucepan, stirring until well blended.
21Bring to a simmer over medium-heat heat.
22Cover and boil for 2 minutes to allow steam to wash any sugar from pan sides.
23Uncover and continue simmering, without stirring, until mixture bubbles loudly and reaches 244-245 on a candy thermometer (about 1 1/2 minutes).
24(To test without a candy thermometer, drop a teaspoon of syrup into ice water; when cooked to the proper temperature the syrup will form a firm ball that holds its shape when squeezed.)
25Immedately remove pan from heat and set aside. With mixer set on medium speed, beat egg whites until very frothy and opaque.
26Raise speed to high and beat until whites just begin to stand in soft peaks; be careful not to overbeat.
27Meanwhile, return syrup to burner and reheat just to boiling.
28Beating whites on high speed, immediately begin pouring boiling syrup in a stream down the side of the bowl (avoid beaters or syrup will stick to them or be thrown onto bowl side), pouring rapidly enough that all the syrup is incorporated in about 15 seconds.
29Add salt and continue beating on high speed until the mixture is stiffened, glossy and cooled to barely warm.
30Beat in vanilla and coffee mixture until evenly incorporated.
31Sift powdered sugar and cocoa onto a sheet of wax paper.
32A bit at a time, whisk into egg-white mixture.
33Whisk in melted chocolate just until smoothly incorporated.
34Frost cake immediately, or store frosting airtight for up to 48 hours.
35Frosting may soften and gradually deflate upon longer standing.