Whisk 2/3 cup all-purpose flour, the coconut flour, sugar, salt, and baking powder in a bowl. Slowly whisk in 2 cups water until smooth. Batter should look like thick pancake batter; if it's too thick, whisk in water, 1 to 2 tbsp. at a time. Let rest at least 20 minutes and up to
Meanwhile, pour oil into a large pot to come 3 in. up sides. Insert a deep-fry thermometer and heat to 350° over medium-high heat (15 to 20 minutes).
Slice baby or red bananas in half lengthwise. If using regular bananas, slice in half crosswise, on the diagonal. Put remaining 2/3 cup all-purpose flour in a bowl and dip bananas to coat thinly. Set a cooling rack on a baking sheet.
Dredge a few banana pieces in batter and carefully drop into oil. Boost heat to maintain an even 350°. Fry bananas until deep golden brown, 7 to 10 minutes; then lift with a slotted spoon to rack. Scoop out and discard bits of fried batter in oil. Repeat with remaining bananas and batter, returning oil to 350° between batches.
Put 2 to 3 banana pieces in each bowl. Top with a scoop of ice cream and a sprinkle of toasted sesame seeds.