This is a very soft sticky rice cake. This recipe yields two 6" round sticky rice cakes or one 8" round cake.
If you use an Instant Pot, you may want to halve the recipe and make just one 6" cake. Otherwise, you can stack two pans in the Instant Pot by either using two Instant Pot Steamer Baskets or try using a pair of chopsticks in between the two pans to create some separation.
In a large bowl, stir together water (or bean cooking liquid - see Notes) and sugar until dissolved. Gradually whisk in rice flour until well blended. Gradually whisk in oil until combined. Add vanilla, adzuki beans and longan, if using. Stir to combine.
Divide batter between two plastic wrap lined 6" round cake pans, or one 8" round pan. Spray one side of oil with cooking oil. Place foil, oiled side down, over cake pan and cover loosely.
This works for 6" round cake pan only. Place steam trivet in bottom of Instant Pot. Add 1 cup water. Place cake pan on trivet. Cover and cook at high pressure for 30 minutes. Let natural release 10 minutes, then quick release pressure.
Set up Chinese steamer. Use two tiers to steam 6" round cake pans or one tier to steam 8" round cake pan. Steam for 1 hour.
To cook adzuki beans, soak 1 cup dried adzuki beans overnight. Then place drained beans in Instant Pot or pressure cooker; add enough water to cover by 1" for 5-7 minutes. Drain beans, reserving cooking liquid. Measure out 3 cups of cooking liquid. Add 1 3/4 cup sugar to cooking liquid and stir to dissolve. Use this in place of the water and sugar in this recipe.