Combine mashed bananas, sliced bananas, 1/4 cup sugar, and salt in a bowl. Let stand for 5 minutes. Add 1 1/2 cup tapioca starch, turmeric, and vanilla extract; mix thoroughly.

Grease a 9-inch steaming insert with coconut oil. Line the bottom with parchment paper. Grease parchment paper with remaining 1 teaspoon coconut oil. Pour banana batter into the insert.

Pour 2 cups water into a multi-functional pressure cooker (such as Instant Pot®). Place the trivet inside. Place the steaming insert on the trivet. Close and lock the lid. Select Steam function. Set timer for 20 minutes.

Unlock and remove lid. Transfer cake carefully to a cooling rack or cutting board. Let cool until firm, at least 1 hour. Run a knife around the sides of the steaming insert to loosen cake; slide onto a serving dish.

Combine coconut milk and 1/4 cup sugar in a small saucepan; bring to a simmer. Mix water and 1 tablespoon tapioca starch together in a small bowl; add to the simmering coconut milk. Continue simmering, whisking constantly, until bubbly and thick, about 5 minutes. Remove from heat.

Cook and stir sesame seeds in a dry skillet over low heat until lightly toasted and fragrant, about 5 minutes. Keep stirring to ensure they don't burn.

Slice cake and serve 1 to 2 tablespoons coconut sauce over each slice. Sprinkle toasted sesame seeds on top.