Stir the oat milk and apple cider vinegar together and let the mixture stand for 5 minutes. Preheat the oven to 350 F, then grease and line two 9-inch round cake pans.
Whisk the flour, sugar, cocoa powder, baking soda, and salt in a large bowl. In another bowl, whisk the soured oat milk, oil, eggs, vanilla, and red food coloring.
Pour the wet ingredients into the dry ingredients and whisk just until the batter is smooth. Divide it evenly between the prepared pans.
Bake for 28 to 32 minutes, until the centers spring back when lightly pressed. Cool the cakes in their pans for 10 minutes, then transfer them to a rack to cool completely.
Beat the plant butter for 2 minutes until light and smooth. Add the powdered sugar gradually, followed by the vanilla and salt, and beat until fluffy.
Add the cool dairy-free cream cheese in small portions, mixing on low just until smooth. Refrigerate the frosting for 20 minutes if it is too soft to spread cleanly.
Place one cake layer on a serving plate and spread it with a generous layer of cream cheese frosting. Add the second layer, then frost the top and sides of the cake.
Refrigerate the assembled cake for 30 minutes to set the frosting. Let it stand at room temperature for 20 minutes before serving.