Prepare the Cake Layers: Preheat oven to 350°F. Grease (with butter) and flour 2 (9-inch) round cake pans. Whisk together hot water and cocoa in a small bowl until smooth; let cool to room temperature, about 15 minutes.

Meanwhile, beat sugar and butter in bowl of a stand mixer fitted with a paddle attachment on medium-high speed until light and fluffy, 2 to 3 minutes. With mixer running on low speed, add eggs, 1 at a time, beating well after each addition. Whisk together buttermilk, baking soda, vanilla, and cocoa mixture in a glass measuring cup. Sift together flour, baking powder, and salt in a medium bowl, and add to butter mixture in 3 additions, alternatively with buttermilk mixture, beginning and ending with flour mixture. Beat on low speed until just blended after each addition. Pour batter into prepared pans, and spread into even layer. Bake in preheated oven  until a wooden pick inserted in center comes out clean, about 30 minutes. Cool in pans on wire racks 10 minutes. Remove from pans to wire racks, and let cool completely, about 1 hour.

Meanwhile, prepare the Fluffy Chocolate Filling: Place whipping cream in large bowl. Sift powdered sugar and cocoa into cream. Beat with an electric mixer on low speed until smooth, about 30 seconds. Increase speed to high, and beat until stiff peaks form, 1 to 2 minutes.

Using a serrated knife, slice cooled cake layers in half horizontally to make a total of 4 cake layers. Place 1 cake layer on serving plate or cake stand; spread top with about 1 cup chocolate filling, spreading to edges using a small offset spatula. Repeat with remaining 3 layers and remaining 3 cups filling. (Do not frost sides of cake. The stacked cake is supposed to resemble stacked brown stones with mortar between the layers.)