Preheat the oven to 350 (F). Grease and lightly flour 2 round, 9-inch cake pans.
In a medium bowl, whisk together 2 1/2 cups flour and 1 teaspoon salt. Set aside.
In a small bowl, use a rubber spatula or the back of a spoon to combine 1 tablespoon cocoa powder, 1/2 teaspoon espresso powder, and 1 1/2 tablespoons red food coloring until a thick paste forms and all the lumps are gone. Set aside. In the bowl of a freestanding mixer with a paddle attachment, combine 2 cups sugar and 1 cup butter until light and fluffy.
Add 2 eggs, one at a time, mixing well after each addition.
Add a third of the flour and salt mixture (from the second step) before adding 1/2 cup buttermilk, mixing until just combined. Add another third of the flour mixture, before adding another 1/2 cup of buttermilk, again mixing until just combined. Finish by adding the remaining third of the flour mixture, mixing until just combined. Be careful not to overmix and just keep mixing until everything is just combined, or your cake will be hard and fudge-y and I will cry for you!
Add the cocoa powder and food coloring mixture (from the third step) until the color is evenly dispersed. Again, mix well, but just until everything is combined.
In a small bowl or ramekin, quickly stir together 1/2 teaspoon baking soda and 1 tablespoon cider vinegar. This will foam and should be put into the batter immediately after being mixed. Blend on medium-high speed until the batter is consistent.
When finished mixing, spoon batter evenly into pans — it will be surprisingly thick, so don't worry! Bake for 25 - 30 minutes, or until the cake springs back when pressed in the center. Let cool in pans on a cooling rack for an additional 15 - 25 minutes, before turning out to cool on the cooling rack completely.