Preheat your oven to 350°F (176°c)
Prepare the (3) 6" cake pans and line each with parchment paper.
Using a mixing bowl - combine flour, cocoa powder, baking soda, baking powder, corn starch, and salt. Whisk together until well combined.
In a bowl or measuring cup, pour liquid ingredients together such as the milk, apple cider vinegar, and coffee together. Whisk thoroughly and set aside.
In a large mixing bowl & using an electric hand mixer or headstand mixer, add the softened butter, sugar, and vanilla. Beat on high speed until it reaches its soft fluffy peaks. This will usually take between 3-5 minutes. You will know it is finished beating when it is nice and airy.
Add the dry ingredients to the whipped butter mixture and oil. Beat together until it is well combined. Then, gradually add liquid ingredients as you mix. The batter will slightly looked curdled or clumpy, which is okay. Keep mixing until all ingredients are well incorporated.
Do not overmix the the batter. Just enough until the dry spots have fully dissolved (or have been hydrated). This is a wet batter, so it should look runny but not too thin.
Prepare your lined cake pans. Evenly divide cake batter to each cake pans. You can also measure the total weight of you cake batter and divide by 3 to get an even amount to each pan (but this is optional).
Bake in the oven for 30 - 45 minutes depending on your oven, or until a toothpick runs clear.
Remove cakes from the oven and allow it to cool for about 5 minutes. Then, remove from the pans / parchment paper and allow to cool on a cooling rack completely. Do not let your cake sit in the pans to cool, as it may cause your cake to stick on the pans.
Be sure to allow your cakes to cool down for at least an hour before handling. This way, cake is firmer and easier to handle. You may also prepare the cake the night before, wrap it, and assemble it the next morning.