Heat oven to 350° F.
Spray 3 8-inch round pans with baking spray. Set aside.
In a small bowl, pour boiling water over chocolate.
Whisk until smooth. Set aside.
In a large bowl, sift together flour, 1 ⅛ cups sugar, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together egg yolks, buttermilk, vegetable oil, and vanilla.
Whisk in chocolate mixture.
Pour over dry ingredients.
Mix well. Set aside.
With an electric mixer, whip eggs and cream of tartar until foamy.
With mixer running, add sugar in small amounts.
Beat to medium-stiff peaks.
Fold into batter.
Portion batter into prepared pans.
Bake for 18-20 minutes, or until done.
Allow to cool completely in pans before removing.