Buy a cake mix and follow the directions. I mean, you could make it from scratch, but that would be very impractical for travelling with all those ingredients.
In a pot add about 2” of water. Place a flat, heat resistant object in the center. I used a rock that I found.
Pour cake batter into a bowl (a plastic camp bowl was sufficient for me).
Place the bowl on the rock and cover.
Steam for about 30 minutes, checking with a toothpick or twig poking the center until it comes out clean
I split the cake mix in half and prepared this dessert twice. Half of the mix was ample amount for 2 people