Coat bottoms of 2 (8-inch) round cake pans with cooking spray (do not coat sides of pan); line bottoms of pans with wax paper. Coat wax paper with cooking spray; set aside.
Beat sugar and butter at medium speed of a mixer until well-blended. Gradually add egg substitute; beat well. Combine flour, cocoa, baking soda, and salt in a bowl. With mixer running at low speed, add flour mixture to sugar mixture alternately with sour cream, beginning and ending with flour mixture. Gently stir in boiling water and vanilla.
Pour batter into prepared pans. Bake at 350° for 35 minutes or until cake springs back when touched in center. Loosen layers from sides of pans using a narrow metal spatula; turn out onto wire racks. Peel off wax paper; let cool.
Place 1 cake layer on a plate; spread with 1/2 cup Chocolate Frosting. Top with other cake layer; spread remaining frosting over cake.
Rich and moist and chocolatey, and wow, that frosting. I used good-quality bittersweet chocolate for a darker flavor. Definitely fit for a special occasion, and definitely worth making again. Next time I might use bourbon or amaretto in place of some of the milk in the frosting.