Mash the banana until smooth. Whisk in the milk and vanilla.
Stir in the oat flour, cocoa powder, baking powder and salt.
Pour batter into a large, greased ramekin (or divide batter into two normal-sized ramekins). For an extra-gooey center, pour in half the batter, place chocolate chips in the middle, and cover with the rest of the batter.
Fill the bottom of your steamer with water and turn on high. Once the water starts boiling, turn the heat down to medium and put your cake into the steamer basket. Steam for 15-20 minutes, or until cake is fairly firm to the touch (if you press it with a spoon, it shouldn't jiggle, unless you want a really undercooked cake). Feel free to under-steam it if you like since this is also really similar to any basic overnight oat recipe, which you can eat raw.