To make cake: Heat oven to 350º. Grease 3 round 9-inch cake pans with shortening or butter and dust with flour. (If you have only 2 cake pans, bake 2 layers at once, then wash a pan and bake the last layer separately.)
In the bowl of a stand mixer, cream butter until smooth. Add sugar and beat 1 minute on medium speed. Add vanilla and eggs, 1 at a time; scrape the bowl and beat 1 minute on high after each egg is added.
Measure cocoa, baking soda, salt, and flour into a mesh wire strainer. Shake the ingredients through the mesh onto a piece of waxed paper, to sift.
Reduce mixer speed to its lowest setting: add 1/3 of the flour mixture, then ½ of the sour cream; scrape bowl (turn off mixer while scraping). Repeat; then add the last third of the flour mixture. Mix just until blended. Pour into boiling water, all at once, and fold gently with a large spatula (to fold, hold the spatula like a knife, cut through the center of the batter, then lift up and over; turn the bowl slightly and repeat).
Pour equal amounts of batter into the 3 pans (either weigh the filled pans or measure the batter’s depth with a ruler).
Bake 35 minutes, or until a toothpick inserted into middle comes out with only a few fudgy crumbs on it.
Cool 10 minutes in the pans. Run a knife around the edges and turn out of pans onto wire racks to cool completely. If necessary, trim the cake layers until they’re level.
Cream cheese icing: In the bowl of a stand mixer, beat together cheese and butter about 2 minutes on high speed. Beat in sugar and vanilla until mixture is smooth and creamy. Set aside until you’re ready to assemble cake.
To assemble: Place 1 layer on cake plate. Carefully spread a layer of cream cheese frosting over layer of cake. Repeat with second layer. It is optional to frost top layer. Frost sides of cake.
Using a large frosting tip, ice a border around the top of the cake. Refrigerate at least 1 hour to firm frosting.
Chocolate ganache: Wait to make the ganache until the cake has been frosted and refrigerated. When ready, in a medium saucepan, warm cream and corn syrup over medium heat. Cover, if necessary, but stir occasionally until mixture begins to bubble. Remove from heat; whisk in chocolate and butter until mixture is creamy and smooth.
When cake’s frosting has hardened, pour about 1 cup chocolate ganache onto the middle of the top layer. (You will have about 1 cup left, which will keep well refrigerated and can be served with ice cream). Gently spin the cake plate, smoothing as necessary with a spatula, to spread ganache to the sides without damaging the border.
If your cake plate has enough room (and you have enough frosting,) use a ribbon frosting tip to ice a small border around the bottom of the cake.
**Note: Grandmama made 1 ½ of the cream cheese icing and only ½ of the chocolate ganache.