If you have a small lump of alum, crush it on a clean stone or in a mortar until fine, then sift through a tea strainer so there are no sharp grains. Measure 3/4 tsp level. Set aside.

Pour milk into the heavy pan and bring to a full boil, then reduce the flame to medium-low. Cook, stirring often, until the milk is reduced to a little more than half of the starting volume. Skim foam if you like a cleaner look.

Sprinkle the measured alum powder evenly over the surface. Stir well once. Continue cooking on medium-low, stirring frequently, scraping any solids that stick to the sides back into the milk. The milk should begin to look grainy and split like soft chenna suspended in whey-forward liquid.

When the mixture looks uneven, use a potato masher or whisk to break large lumps so the texture is more even. Do not turn it into a paste; you want small granules.

Add sugar. Stir until the sugar dissolves completely. Keep cooking until the sugar syrup looks almost dry and the mass is thick and moving slowly.

Add ghee 1 tablespoon at a time, stirring well after each addition, until both tablespoons are incorporated and the mixture pulls away from the sides in a cohesive, fudge-like mass.

Continue on low heat until the colour is pale to light golden and the mixture looks almost dry but still slightly glossy in spots. It will feel heavy under the spoon. Do not wait for deep brown on the flame; colour continues during the rest step.

Immediately scrape into the greased tin. Level the top gently. Cover tightly with foil, then wrap the whole tin in a thick towel and leave undisturbed at room temperature about 4 hours so residual heat finishes the centre and deepens the colour.

Unmould carefully. Cut into about 20 squares or diamonds with a sharp knife wiped with a little ghee between cuts. Garnish with pistachio slivers and optional rose petals.

Store in an airtight container at room temperature up to 4 to 5 days in a cool kitchen.