Bring 6 cups (1.5 L) whole milk to a gentle boil in a heavy pot. Switch off heat.

Stir lemon juice with warm water. Add to the hot milk in a thin stream, stirring gently. Let stand 2 minutes until the milk splits and whey looks pale green-yellow.

Line a colander with cheesecloth. Pour curds and whey through. Rinse lightly with cool water to remove lemon tang.

Gather the cloth and hang 12 to 15 minutes so excess whey drips off. Open the cloth. You should have moist, crumbly chena, about 1 1/4 to 1 1/2 cups packed.

Bloom saffron in 1 tablespoon warm milk. Set aside.

Line an 8×8 inch pan with parchment, leaving overhang on two sides.

Place the same wide heavy pan on medium-low. Crumble in chena. Add reserved 2 cups whole milk, sugar, milk powder, cardamom, saffron milk, and salt if using. Stir until the mixture looks loose and milky, about 3 to 4 minutes.

Taste a tiny cooled dab on a spoon if you like less sweetness. Adjust sugar now only; do not add liquid sweeteners later.

Raise heat to low-medium. Cook, stirring often and scraping the pan bottom, until the mixture thickens and coats the spatula, about 12 to 18 minutes. It will still look pourable at first.

Continue on low-medium, stirring continuously, until the mass pulls away from the sides and a spatula pressed flat on the surface lifts away clean with no wet streak, about 15 to 22 minutes more (35 to 45 minutes total cook after step 7).

Immediately scrape into the lined pan. Spread evenly with an offset spatula. Tap the pan once on the counter to release air pockets.

Sprinkle pistachios and rose petals on top while still warm. Press gently so they stick.

Cool on the counter 20 minutes, then refrigerate uncovered until firm enough to slice, at least 2 hours (3 hours for cleaner edges).

Lift out using parchment overhang. Wipe the knife clean between cuts. Cut into 18 squares.