Golang base struct to define methods in substructs

I would like to create a base struct which have to method, I want to use these methods in the substructs. For example:

type Base struct {
  Type string `json:"$type"`
}

func (b Base) GetJSON() ([]byte, error) {
  return json.Marshal(b)
}

func (b Base) SetType(typeStr string) interface{} {
  b.Type = typeStr
  return b
}

In the new struct I want to use it like this:

type Auth struct {
  Base
  Username
  Password
}

and call these methods in the main:

func main() {
  a := Auth{
    Username: "Test",
    Password: "test",
  }
  a = a.SetType("testtype").(Auth) 
  j, _ := a.GetJSON()
}

In the SetType case I got a panic caused by interface{} is not Auth type, it is Base type. In the GetJSON case I got a json about the Type, but only the Type.

Is there any solution for the problem what I want to solve?