Prepare the Custard Filling: In a saucepan, whisk together milk, heavy cream, sugar, and egg over medium heat until thickened. Stir in cornstarch and cook until smooth. Remove from heat and fold in mashed banana and vanilla extract. Let it cool.
Make the Sponge Cake: Preheat oven to 350°F (175°C). Beat eggs and sugar until fluffy. Add milk, vanilla, oil, and mashed banana. Sift in flour, baking powder, and salt, then mix until smooth. Pour batter into a parchment-lined baking sheet and bake for 10-12 minutes. Let it cool.
Shape the Cakes: Once the cake has cooled, use a cookie cutter or knife to cut out small banana-shaped pieces.
Assemble the Tokyo Banana: Place a spoonful of custard filling in the center of each cake piece. Fold the cake over the filling to form a banana shape.
Chill and Serve: Refrigerate the assembled Tokyo Banana for about 1 hour to set the filling. Serve and enjoy!