Soak the banana leaves in warm water for about 30 minutes to soften them. Cut them into squares (about 12x12 inches) and set aside.
In a large mixing bowl, combine the cornstarch, sugar, and salt.
In another bowl, mix the milk, coconut milk, melted butter, and vanilla extract.
Add the liquid mixture to the dry ingredients and mix well until you get a smooth batter.
Place a banana leaf square on a flat surface and spoon some of the batter onto it. Add some raisins and chopped nuts on top of the batter.
Fold the banana leaf to form a packet, making sure that the batter is fully enclosed.
Repeat until all the batter is used up.
In a large pot, bring water to a boil. Add the tamal packets and cook for about 30-45 minutes or until the batter is firm and the tamal is fully cooked.
Remove the tamal packets from the pot and let them cool for a few minutes before serving.