Take khoya and milk in a pan on medium heat. Mix and cook with stirring constantly until it becomes thick, no more bubbles from moisture and leaves the sides of the pan. It may take 10-20 minutes. This time will widely vary depending on how moist your khoya is.
Remove it to a plate and keep it aside.
In the same pan, add mango pulp and sugar. Mix and cook on medium heat with stirring constantly until it thickens like a super thick pulp (not runny at all). It will coat the spatula or spoon. It takes around 15 minutes.
Add cooked khoya and cardamom powder to the mango pulp. Mix well.
Add milk powder. Mix well.
Keep stirring and cooking. Within 8-10 minutes the mixture will thicken and comes together like a dough and leaves the pan.
Remove it to a plate and let it cool down to touch.
Grease the modak mould using ghee and shape it into modak. Repeat the same for the rest.
Notes
Do not overcook the modak mixture otherwise, it turns hard and chewy.
Adjust the sugar quantity depending on the sweetness of your mango. If using canned already sweetened pulp then you’ll need less sugar.
The color of the mango modak depends on the type and color of your mango.
If the modak mixture is still loose and doesn’t hold the shape then you can add a little more milk powder and mix.