Wales manager Craig Bellamy recently hailed the former Nottingham Forest man as an “elite” player, adding that Johnson is a “ridiculous talent”.
Earnshaw hopes Johnson can realise his potential to be the “superstar” in the Wales side.
“I’m a big fan of Brennan Johnson, I think he’s brilliant,” added Earnshaw.
“What I really love is that he’s getting a result at the end of his play because sometimes his play is pace, his work-rate is brilliant and there was not that end result in goals in that last action.
“What I’m seeing right now is great finishing, great movement and, yes, I really want to see that in the Wales shirt.
“I think he’s a high-level talent and he needs to find that consistency; right now he’s doing that for Tottenham.
“I think he needs to realise ‘do you know what, I can be the superstar in the Wales team’ and be doing that for club and country.
“I’m happy for him, he’s been on fire, that’s six in six, unbelievable.”