Slice the banana thinly.
Mix milk, whipping cream, and sugar in a bowl.
Cut castella cake into bite-sized pieces.
Place a layer of castella cake at the bottom of a glass dish.
Add banana slices on top of the cake.
Pour the cream mixture over the bananas.
Repeat this process 2–3 times.
Decorate with additional banana slices and biscuits on top at the end.
Let it chill in the refrigerator for at least 1 hour.