When you have a struct implementing an interface, a pointer to that struct implements automatically that interface too. That’s why you never have *SomeInterface in the prototype of functions, as this wouldn’t add anything to SomeInterface, and you don’t need such a type in variable declaration (see this related question).
An interface value isn’t the value of the concrete struct (as it has a variable size, this wouldn’t be possible), but it’s a kind of pointer (to be more precise a pointer to the struct and a pointer to the type). Russ Cox describes it exactly here :
Interface values are represented as a two-word pair giving a pointer
to information about the type stored in the interface and a pointer to
the associated data.
This is why Interface, and not *Interface is the correct type to hold a pointer to a struct implementing Interface.
So you must simply use
var pi Interface