Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line the bottoms with parchment paper and lightly grease the sides.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined and lump-free.
2 1/2 cups all-purpose flour, 2 tablespoons unsweetened natural cocoa powder, 1 teaspoon baking soda, 1 teaspoon fine sea salt
In a large mixing bowl, beat the butter, oil, and granulated sugar on medium speed for 2–3 minutes, until light and fluffy. Scrape down the sides as needed.
1 1/2 cups granulated sugar, 1/2 cup unsalted butter, 1/2 cup neutral oil
Beat in the eggs one at a time, mixing well after each addition, until the batter looks smooth and slightly thick.
3 large eggs
In a small bowl or measuring cup, whisk together the buttermilk, vanilla, vinegar, and red gel food coloring. Start with 1 tablespoon of gel color; you can add more later for a deeper red.
1 cup buttermilk, 2 teaspoons pure vanilla extract, 1 teaspoon white vinegar, 1-1 1/2 tablespoons red gel food coloring
With the mixer on low, add about one-third of the flour mixture, then half of the buttermilk mixture, another third of the flour, the remaining buttermilk, and finish with the remaining flour. Mix just until combined after each addition to avoid overmixing.
Check the color of the batter. If you want a stronger red, gently fold in a bit more red gel coloring by hand with a spatula. The batter should be thick but pourable and glossy.
1-1 1/2 tablespoons red gel food coloring
Divide the batter evenly between the prepared pans, smooth the tops, and tap the pans gently on the counter to release air bubbles. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out mostly clean with a few moist crumbs.
Let the cakes cool in the pans for 10 minutes. Run a thin knife around the edges, turn them out onto a wire rack, remove the parchment if used, and let cool completely (about 1 hour).
In a large bowl, beat the softened cream cheese and butter together on medium speed for 2–3 minutes until very smooth and creamy. Add vanilla and salt and mix to combine. Gradually add powdered sugar, 1 cup at a time, beating on low then medium until thick, fluffy, and spreadable.
16 oz cream cheese, 1 cup unsalted butter, 4-5 cups powdered sugar, 2 teaspoons pure vanilla extract, 1/4 teaspoon fine sea salt
Level the tops of the cooled cake layers if needed. Place one layer on a serving plate and spread a generous layer of cream cheese frosting on top. Place the second layer on top, bottom side up for a flat surface. Apply a thin crumb coat of frosting over the whole cake and chill for 15–20 minutes, then apply a final, thicker layer of frosting and smooth or swirl as desired.
Decorate as desired with extra cake crumbs, sprinkles, or fresh berries. Slice with a sharp, thin knife, wiping it between cuts for neat layers. Serve at room temperature.