Spray a 7 inch springform pan with flour-added baking spray. Alternatively, you can just grease it with butter or shortening.
In the bowl of a stand mixer or in a regular mixing bowl, combine 216 grams cake mix, 110 grams mayonnaise, 70 grams beaten egg and 105 grams water. Beat for about 3 minutes, scraping the sides of the bowl once or twice. Batter will lighten in color.
Scrape mixture into your cake pan. Cover the cake pan loosely with a sheet of foil. This is to prevent steam from falling and condensing on the cake. You don't have to make a tight seal, you are basically just creating a little umbrella.
Meanwhile, boil 1 ½ cups of water in the microwave. Put the water in the instant pot, then set the cake pan on the trivet and lower it into the Instant Pot.
Close the pot, put the sealing valve in the middle position and set the timer for 30 minutes. When time is up, allow for a 15 minute release. Release valve, remove lid from Instant Pot and carefully lift the cake. It should have risen quite a bit. The top might crack a little, but this is okay.
Allow the cake to cool in the pan for about 10 minutes, then slide a knife between the pan and the cake to loosen. Remove sides. Let cool for another 20 to 30 minutes. When ready to frost, you can cut horizontally to make thin layers or leave as one thick layer.
Wait for the cake to cool before icing.