What blessing is recited over instant mashed potatoes made from potato powder?

Shalom,

Potatoes that are crushed and dried until they become like flour, and then water is added to them and they become like regular mashed potatoes, one should recite ha'adamah over them, both according to the custom of the Sephardim and according to the custom of the Ashkenazim.

Sources: See Halacha Berurah (Siman 204, seif 25).

With blessings,

Hillel Meirs