Take the rasgullas out of the syrup, squeeze each one gently to remove extra liquid, and pat dry with a clean towel. Cut half into small pieces and keep the rest whole or halved for the top. Toss the cut pieces in 1 tablespoon of flour.
Preheat the oven to 175°C (347°F). Grease a 7-inch tin and line it with parchment paper.
In a large bowl, whisk the sugar, oil, eggs and vanilla for 2 minutes until smooth. Add the milk and 2 tablespoons of rasgulla syrup, and mix well.
Sift in the flour, baking powder, baking soda, cardamom and salt. Fold gently until just combined.
Fold in the chopped rasgulla pieces with a light hand.
Pour the batter into the tin and tap lightly on the counter. Press the reserved rasgulla halves gently into the top and sprinkle with pistachios.
Bake for 35 to 40 minutes, until a toothpick comes out clean. Start checking at 30 minutes, and test in a spot away from a rasgulla piece.
Rest in the tin for 10 minutes, then lift out and cool completely on a wire rack.
Brush the top lightly with the saffron milk and add dried rose petals if you like.