Preheat the oven to 350 degrees F. Butter the bottoms and sides of three 8-inch round cake pans. Line the bottom of each pan with a round of parchment or waxed paper and butter the paper.
Sift the flour, sugar, cocoa powder, baking soda, and salt into a large bowl. Whisk to combine them well. Add the oil and sour cream and whisk to blend. Gradually beat in the water. Blend in the vinegar and vanilla. Whisk in the eggs and beat until well blended. Scrape down the sides of the bowl and be sure the batter is well mixed. Divide among the 3 prepared cake pans.
Bake for 30 to 35 minutes, or until a cake tester or wooden toothpick inserted in the center comes out almost clean. Let cool in the pans for about 20 minutes. Invert onto wire racks, carefully peel off the paper liners, and let cool completely. (Deb note: These cakes are very, very soft. I found them a lot easier to work with after firming them up in the freezer for 30 minutes. They’ll defrost quickly once assembled. You’ll be glad you did this, trust me.)
To frost the cake, place one layer, flat side up, on a cake stand or large serving plate. Spread 2/3 cup cup of the Peanut Butter Frosting evenly over the top. Repeat with the next layer. Place the last layer on top and frost the top and sides of the cake with the remaining frosting. (Deb note 1: Making a crumb coat of frosting–a thin layer that binds the dark crumbs to the cake so they don’t show up in the final outer frosting layer–is a great idea for this cake, or any with a dark cake and lighter-colored frosting. Once you “mask” your cake, let it chill for 15 to 30 minutes until firm, then use the remainder of the frosting to create a smooth final coating. Deb note 2: Once the cake is fully frosting, it helps to chill it again and let it firm up. The cooler and more set the peanut butter frosting is, the better drip effect you’ll get from the Chocolate-Peanut Butter Glaze.)
To decorate with the Chocolate–Peanut Butter Glaze, put the cake plate on a large baking sheet to catch any drips. Simply pour the glaze over the top of the cake, and using an offset spatula, spread it evenly over the top just to the edges so that it runs down the sides of the cake in long drips. Refrigerate, uncovered, for at least 30 minutes to allow the glaze and frosting to set completely. Remove about 1 hour before serving. Decorate the top with chopped peanut brittle.