Combine buttermilk and vinegar and stir. Set aside.
In a bowl, combine all-purpose flour, cocoa powder, baking soda, and salt. Stir very well to blend.
In a large mixing bowl, beat together with an electric mixer butter and sugar for 2-3 minutes, until creamy.
Add oil and beat again to combine.
Add the eggs one by one, beating after each addition until combined.
Scrape well the sides of the bowl.
Add some food coloring and mix gently with a rubber spatula. The batter should have an intensive red or magenta color.
Gradually add the dry ingredients, alternating with the buttermilk, mixed with vinegar.
Mix gently with the rubber spatula until both are absorbed, and the batter is uniform.
Pour the batter into the prepared pan.
Cover the top of the pan with a piece of paper towel and wrap it tightly with aluminum foil. Make sure there is room for the cake to rise, so keep the foil and the paper loose.
Pour 3/4 cup water into the Instant Pot. Use the trivet with the handles up to place the cake pan into the Pot.
Close the lid and make sure the valve is in the Seal position. Select the “Manual” setting and dial 40 minutes on high pressure.
When the program is over, let the pressure release naturally for 15 minutes. Release any remaining pressure manually and open the lid. Remove the pan from the Instant Pot, holding the trivet handles with kitchen mittens. Test the cake for doneness by inserting a toothpick in the middle: if it comes out clean, the cake is made. If it comes out with some dough, wrap the pan again with a paper towel and aluminum foil and cook for another 10 minutes. Let the pressure release naturally for 10 minutes.
Remove the cake from the pan and let it cool until room temperature before assembling it.