Pour 1 1/2 cups water into the Instant Pot and place trivet in pot. Coat a round 6- or 7-inch cake pan with cooking spray and set aside.
Prepare cake batter according to package directions. Divide between two same-size bowls and pour the batter from one of the bowls into the prepared pan. Cover with foil and place pan on the trivet in the Instant Pot.
Place the cover on the pot and set timer to 35 minutes on high pressure (cake setting). It will take the IP around 6 minutes to build up pressure before it begins cooking.
When cooking is done, let the pressure release naturally for 10 minutes then do a quick release and allow the float valve to drop.
Remove cover and check if cake is done by sticking a toothpick into the center. Remove pan from the Instant Pot and let cool for 5 minutes. Carefully invert the cake and remove from pan. Let cool on a wire rack. Cut rounded top off cake. Repeat process with remaining batter.
Spread frosting over the top of one cooled cake, top with the remaining cake and remaining frosting. Cover and refrigerate until ready to cut in slices and serve!