For the vanilla batter: Preheat the oven to 350 degrees F. Generously spray a 12-cup Bundt pan with the baking spray.
Combine the butter, sugar, salt, baking powder, vanilla and oil in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed for 4 minutes. Stop and scrape the bowl and paddle with a rubber spatula. Add 2 of the eggs at a time, beating on medium speed for 30 seconds after each addition. Scrape the bowl.
Add half of the cake flour and mix on low speed just until combined. Add the cream and buttermilk and mix again on low speed just until combined. Scrape the bowl.
Add the remaining cake flour and start mixing on low speed to combine, then increase the speed to medium and beat for 4 minutes.
For the chocolate batter: Transfer 2 cups of the vanilla batter to a medium bowl. Add the cocoa, sugar and baking powder and stir with a fork or small whisk until completely combined. Add the hot water and stir slowly until completely combined.
Pour about half of the vanilla batter into the prepared pan. Dollop about half of the chocolate batter on top. Repeat with the remaining vanilla and chocolate batters. Run a skewer or butter knife through the pan to slightly swirl the 2 batters together.
Bake until a toothpick inserted near the center comes out with a few moist crumbs, 65 to 70 minutes. The cake will rise to fill the pan and will settle slightly once removed from the oven. Let the cake cool in the pan for about 30 minutes then turn it out onto a wire rack to cool completely.
The flavor and texture of this cake intensifies and improves after sitting overnight.
Using a baking pan spray that contains flour is the secret to ensuring that your Bundt cakes unmold easily from the pan.