You choose a target as you cast Whammy Burn, before revealing any cards from your whammy deck.

You can’t draw cards from the whammy deck or interact with it in any way other than by effects that specifically interact with the whammy deck. Effects that affect cards you own not on the battlefield don’t affect cards in the whammy deck.

The whammy deck exists in the additional deck zone. Since this is a game zone, it’s not outside the game (for effects such as that of Frontier Explorer).

If you don’t have a whammy deck when an effect refers to it, shuffle a Plains, Island, Swamp, Mountain, and Forest card from outside the game to form your whammy deck immediately before performing any part of that effect.