Preheat the oven to 325 degrees F. Spray two 8-inch cake pans with cooking spray and line with parchment paper. Spray again. Set aside.
In a stand mixer fitted with the paddle attachment, combine flour, sugar, cocoa powder, baking soda, baking powder, and salt. Mix on low until evenly combined.
In a small bowl, whisk together eggs, buttermilk, hot water, vanilla extract, and vegetable oil until evenly combined.
With the mixer on low speed, pour the liquid ingredients into the dry ingredients and mix just until combined. Scrape the bottom of the bowl to ensure even mixing.
Divide the batter between the prepared pans and bake for 30-33 minutes. Check for doneness with a toothpick, aiming for a few crumbs. Cool in the pans for 15 minutes, then transfer to a rack to cool completely.
Level the cake tops if needed, wrap in plastic, and freeze for frosting.
For the Filling:
In a saucepan over medium heat, whisk together milk and flour. Cook until thickened, resembling a very thick pudding. Remove from heat and cool to room temperature.
In a stand mixer with the paddle attachment, cream butter and sugar until light and fluffy. Scrape down the sides.
Switch to the whisk attachment, add the cooled milk mixture. Beat until light and fluffy. Add vanilla and salt, then mix on high for 7-8 minutes until smooth.
For the Ganache:
Heat cream until hot and slightly bubbly in the microwave-safe bowl. Add chocolate, let it sit for 3 minutes, then stir until smooth.
Allow ganache to firm up to a spreadable consistency.
You want the ganache to start to firm up to resemble a butter spread. You can either let it sit at room temperature, or line a sheet pan with parchment paper, and even spread the ganache on top and place it in the freezer for no more than minutes! Then you are ready to frost.
Assembly:
Ensure cake layers are completely cooled. Level if necessary.
Place the first layer bottom side down and spread a generous layer of creme filling.
Repeat with the second layer bottom side up.
Apply ganache around the cake with an offset spatula, scraping excess with a cake scraper.