Preheat your oven to 325°F. Spray the bottom of two 9-inch round cake pans, line with parchment paper, spray again, getting the sides and the bottom of the bowl.
In a large bowl, whisk together the cake flour, cocoa powder, and salt.
In a bowl of a stand size mixer fitted with the paddle attachment, cream together the butter, sugar, and vegetable oil until light and fluffy. About 3-4 minutes.
With a rubber spatula, scrape down the bottom and the sides of the bowl. Add eggs one at a time, beating for on medium speed for 30 seconds after each addition. Scrape between each addition.
In another bowl, combine buttermilk, white distilled vinegar, and baking soda. Set aside
Add the dry ingredients to the butter mixture alternately with the buttermilk mixture, beginning and ending with the dry ingredients. Mix until just combined.
With a rubber spatula, gently fold in the sour cream until incorporated.
Divide the batter evenly between the prepared pans and smooth the tops.
Bake for 35-40 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely. You can place the cakes in the fridge to help speed up the cooling process.
While the cakes are cooling, start making the frosting.
For The Frosting
In a heatproof bowl, melt the semisweet chocolate and corn syrup, together. You can do this by placing the bowl over a pot of simmering water (double boiler) or by using a microwaveable safe bowl and doing short bursts in the microwave.(around 30 seconds and stirring) Allow it to cool slightly.
In a large bowl, using a hand mixer, or in a stand size mixer fitted with the paddle attachment, beat the softened butter until creamy. About 2 minutes.
With a rubber spatula, scrape down the sides and the bottom of the bowl. Add powdered sugar, cocoa powder, vanilla and salt. Beat until smooth. About 2 minutes
Add chocolate and corn syrup mixture together, Mix until well combined. Continue beating on medium speed until the frosting is smooth and creamy. About 1-2 minutes
If the frosting is too thick, you can adjust the consistency by adding a dash of cream.
Assembly
Ensure both cake layers are completely cooled to room temperature. If needed, level the cakes by trimming the tops for an even surface.
Place one cake layer, top side up, on the serving plate or cake stand.
Spoon a generous amount of chocolate frosting onto the center of the first cake layer. Use a spatula to spread the frosting evenly to the edges, covering the top of the cake.
Place the second cake layer, top side down, over the frosting for a flat and stable surface.
Apply a thin layer of frosting over the entire cake to seal in crumbs (crumb coat). Chill the cake in the refrigerator for 15-30 minutes to set the crumb coat.
Apply a thicker layer of chocolate frosting over the crumb coat, ensuring the entire cake is evenly covered.
Use a spatula or cake scraper to smooth the frosting on the sides and top of the cake.
If desired, chill the cake in the refrigerator for a short time before serving for a firmer frosting.