It's one of those ideas that's so simple, yet so beautiful that you think - Why didn't I think of that?!
But I didn't! So I'll just follow the trend instead...
I made this one for my niece's birthday. It's not strictly a "cookie cake" because it's layers of cake - vanilla pound cake - not cookie.
Same same, but different... 😉
(Another reason to make the cake in advance!)
Once baked, let the cake cool down, then wrap it in a double layer of cling-wrap and freeze.
Cake doesn't take long to defrost, but I don't wait for that, anyway - preferring to slice it when it's semi-thawed.
But you could use this delicious cream cheese pound cake recipe, or this hot milk vanilla sponge.
Use a plain round-tip icing nozzle, e.g. an Ateco 806, to pipe on your buttercream ...
try get your lines straighter than mine!
Place another layer of cake on top, and repeat piping rounds.
Decorate with meringue kisses, macarons, chocolates, berries, edible flowers, etc.