For Butterscotch Sauce:

In a heavy bottom non stick pan take the sugar and let it melt on low flame. Once it starts melting take it off the gas and add the butter, cream and milk and mix properly. The flame should be constantly on low so that the sugar does not burn. Take of the flame and mix for sometime till it cools down to room temperature. you can store this sauce for upto a month in an air tight bottle in fridge.

For Lassi:
Blend the curd, sugar, butterscotch sauce, butterscotch essence and 1 tbsp cream/malai using  blender.
Add the crushed ice to the lassi and pour in a glass.
Serve immediately.