To make the puff: Combine water, milk, vanilla (if using), sugar, and unsalted butter in a saucepan. Bring to a boil. Remove from heat and stir in the flour until a dough forms. Let it cool slightly, then gradually beat in the 5 beaten eggs until well combined and smooth. Pipe the dough into small puffs on a baking sheet. Bake the puffs at 350°F (175°C) for about 30 minutes or until golden and firm.
To make the saffron infused custard: Combine 1 1/4 cup milk, 2 egg yolks, 1/4 cup sugar, and 2 1/2 Tbsp cornstarch in a saucepan. Cook over medium heat, stirring constantly, until the mixture thickens. Remove from heat, infuse with saffron (if desired), and let cool. Make it a day ahead and refrigerate so that it thickens up completely.
Assembly: Once the puffs are cooled and the custard is chilled, slice the puffs open. Cut the 5 Rasmalai pieces into small pieces to fit inside the puffs. Place a few pieces of rasmalai into the opened puff, top with a generous spoonful of the saffron-infused custard, and then gently close the puff. Serve them chilled and with love.
