Amelia and Scott partied in Miami to toast -- with sparkling cider, we're sure -- her getting one year closer to legal drinking age. They hit up Papi Steak restaurant Saturday night where Scott rolled out a massive five-tier cake.

The cake was gold and shiny -- much like her outfit -- and it was layered with photos of her in her underwear and a couple of others with her and Scott getting cozy. But, that was just the beginning.

Scott also shelled out a diamond-encrusted cross necklace, which she seemed pretty moved by ... based on the fact she was almost in tears when he put it around her neck.

The pair later hit up LIV nightclub ... where Scott reportedly made a quick appearance at the DJ's booth before calling it a night there.

The celebration comes nearly 1 month after she celebrated Scott's 38th birthday by giving him a Harley.