Preheat the oven to 350°F. Center an oven rack in the middle position. Line the bottom of 2 9-inch round cake pans with parchment paper. Lightly spray with nonstick cooking spray.
In a large bowl, sift the flour, sugar, cocoa, baking soda, and salt.
Using a handheld mixer or whisk, mix in the water, buttermilk, oil, eggs, and vanilla until well combined. Spread the batter evenly between the pans.
Bake 28 to 35 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs. Let cool 2 to 3 minutes before running a knife around the edges and turning out onto a cooling rack. Let cool completely.
On a cake stand or serving platter, place 1 cake layer. Spread the frosting evenly over the top. Place second layer on top and spread remaining frosting all over the top and sides. Refrigerate at least 30 to 60 minutes before slicing. Enjoy with a cold glass of milk!
Notes
Storing: Leftover cake can be stored tightly covered in the refrigerator for up to 5 days.Make Ahead & Freezing: Cover each baked and cooled cake layer individually in plastic wrap and refrigerate up to 2 days or freeze up to 3 months. The chocolate buttercream can be made 2 to 3 days in advance (cover and keep in the fridge). Bring cake layers and frosting to room temperature before assembling. Frosted cake can be frozen up to 3 months, simply thaw overnight in the refrigerator and bring to room temperature to serve.Pan Alternative: This cake bakes well in a 9x13-inch pan. Bake times are similar to the time listed above for an aluminum pan. If using a glass pan, reduce the oven temperature to 325°F and keep a close eye on time.Espresso Powder/Coffee: For a more intense chocolate flavor, feel free to add 1 to 2 teaspoons espresso powder. Hershey's original recipe uses 1 cup freshly brewed coffee in place of the hot water. Cocoa Powder: Natural cocoa powder in the cake will give you the rise and texture that makes this cake so good. Dutch-process cocoa will give you a much denser cake. Recipe credit: Recipe adapted from Hershey's.