Place water and trivet inside Instant Pot®.

Grease a (7-inch) cake pan.

Arrange pineapple slices in a single layer on bottom of cake pan, breaking slices in half to fit, as needed. Reserve pineapple juice.

Place cherries in openings of pineapple slices and any extra around slices.

Sprinkle brown sugar over pineapples and cherries.

Cut 2 tablespoons butter into small chunks and arrange over brown sugar.

With an electric mixer, cream together remaining 1/2 cup butter and granulated sugar. Mix until fluffy, 2 minutes.

Add eggs and egg yolk, one at a time, beating after each addition.

Mix in vanilla.

In a separate bowl, combine flour, baking powder, and salt. Mix.

Slowly mix in flour mixture and buttermilk into wet mixture, alternating between flour and buttermilk until ingredients are fully combined.

Pour cake batter into cake pan.

Cover cake pan tightly with foil.

Create a foil sling and carefully lower pan into Instant Pot®.

Close lid and set pressure release to sealing.

Press manual or pressure cook button and set to 32 minutes on high pressure.

Quick release pressure and remove lid.

Carefully remove cake pan from Instant Pot®. Remove foil.

Use a straw to poke 20 holes in cake. Pour reserved pineapple juice on top of cake. Let sit on a cooling rack, 30 minutes.

Run a knife around edges of cake. Turn cake over onto a cake plate to remove from pan.

Let cool to room temperature and serve.