Step 1: Heat 4 cups of full-fat milk in a heavy-bottomed pan over medium heat. Once it comes to a rolling boil, reduce heat to low and add lemon juice or vinegar, stirring gently until the milk curdles completely.
Step 2: Line a colander with a muslin cloth or cheesecloth and drain the curdled milk (chenna). Rinse the chenna under cold water for 1-2 minutes to remove any lemon taste or acidity. Gather the cloth and squeeze out excess water. Hang it for 30-45 minutes to let more water drain, but ensure it remains slightly moist.
Step 3: Transfer the chenna to a plate. Knead it vigorously with the heel of your palm for 8-10 minutes until it becomes smooth and pliable, like a dough. Divide the chenna into 12-15 equal portions and gently roll each into a smooth, flat disc or oval shape.
Step 4: In a large, wide pot, combine 4 cups of water and 1 cup (200g) of granulated sugar. Bring the mixture to a rolling boil over high heat, ensuring the sugar dissolves completely. Add the crushed cardamom pods.
Step 5: Gently slide the prepared chenna discs into the boiling sugar syrup. Cover the pot and cook on high heat for 10-12 minutes. The discs will expand significantly. Do not overcrowd the pot; cook in batches if necessary.
Step 6: While the chenna discs are cooking, prepare the rabri (thickened milk). In another heavy-bottomed pan, heat the remaining 2 cups of full-fat milk. Bring to a boil, then reduce heat to low. Add the remaining 1/2 cup (100g) granulated sugar and saffron threads. Simmer, stirring occasionally, until the milk thickens and reduces to about half its original volume (approx. 15-20 minutes).
Step 7: Once the chenna discs are cooked, carefully remove them from the sugar syrup using a slotted spoon. Gently press each disc between your palms to release excess syrup, then immediately transfer them into the warm thickened milk (rabri).
Step 8: Allow the rasmalai to cool to room temperature, then cover and refrigerate for at least 4-6 hours, or preferably overnight, to allow the flavors to meld and the discs to fully absorb the rabri.
Step 9: Just before serving, garnish with chopped pistachios and almonds. Serve chilled.