In a small bowl, whisk the custard powder with half a cup of the milk until smooth with absolutely no lumps. This step ensures a silky finish. Meanwhile, in a saucepan over medium heat, warm the remaining 2.5 cups of milk. Stir frequently to prevent scorching.

Once the milk just begins to simmer, stir in the sugar until it dissolves completely. Taste and adjust sweetness. Lower to medium-low before adding the custard mixture.

Slowly pour the custard slurry into the simmering milk while whisking continuously. Keep stirring on low-medium heat until the custard thickens to a coat-the-spoon consistency, about 5–7 minutes. The custard should be smooth, thick, and velvety—perfect for coating the back of a spoon.

Remove from heat and stir in the crushed cardamom powder for that warm, aromatic spice. Let the custard cool slightly, then cover the surface with plastic wrap to prevent a skin from forming. Refrigerate for at least an hour—longer if serving chilled.

Place the jamuns in individual serving bowls or one large dish. Spoon the chilled custard generously over them, ensuring each jamun is partially submerged. Garnish with crushed pistachios for texture and visual appeal. Serve immediately or chill further—this dessert shines both warm and cold.