Grease (or spray with non-stick cooking spray) three 8" or 8 1/2" round cake pans.
In a small, heavy saucepan over lowest heat, melt chocolate, stirring constantly until smooth; be careful not to scorch.
Sift together flour, cocoa powder, baking powder, baking soda and salt onto a sheet of wax paper.
In a large mixer bowl, with mixer set on medium speed, beat oil, butter and sugar until very well blended and fluffy.
Beat in chocolate.
One at a time, beat in egg, then whites, coffee and vanilla until smoothly incorporated.
Gently stir half of the dry ingredients, then yogurt into the mixture just until mixed.
Stir in remaining dry ingredients just until well blended and smooth.
Divide 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.
Transfer pans to racks and let stand until completely cooled.
Layers may be wrapped airtight and frozen for later use.
Let return to room temperature before using.
Or frost cake and serve immediately, if desired.
FLUFFY CHOCOLATE FROSTING: Place egg whites in a large mixer bowl.
Set the bowl in a large bowl of very hot tap water and let stand for 10 minutes, stirring occasionally.
In a small heavy saucepan set over low heat, melt chocolate, stirring constantly, until smooth; be very careful not to scorch.
Set aside to cool slightly.
Combine the corn syrup, 1/4 cup water and the sugar in a 2-quarter saucepan, stirring until well blended.
Bring to a simmer over medium-heat heat.
Cover and boil for 2 minutes to allow steam to wash any sugar from pan sides.
Uncover and continue simmering, without stirring, until mixture bubbles loudly and reaches 244-245 on a candy thermometer (about 1 1/2 minutes).
(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.)
Immedately remove pan from heat and set aside. With mixer set on medium speed, beat egg whites until very frothy and opaque.
Raise speed to high and beat until whites just begin to stand in soft peaks; be careful not to overbeat.
Meanwhile, return syrup to burner and reheat just to boiling.
Beating 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.
Add salt and continue beating on high speed until the mixture is stiffened, glossy and cooled to barely warm.
Beat in vanilla and coffee mixture until evenly incorporated.
Sift powdered sugar and cocoa onto a sheet of wax paper.
A bit at a time, whisk into egg-white mixture.
Whisk in melted chocolate just until smoothly incorporated.
Frost cake immediately, or store frosting airtight for up to 48 hours.
Frosting may soften and gradually deflate upon longer standing.