Johnny Manziel -- My Bday Cake ... IS MONEY!!

Johnny Manziel's birthday cake game just went NEXT LEVEL ... 'cause the guy celebrated his big 22 with a 45-pound MONEY CAKE ... TMZ Sports has learned.

Johnny's friends hit up Colossal Cupcakes in Cleveland -- which has baked for stars like Kyrie Irving and Joe Haden -- and ordered up a custom dessert for the new Cleveland Browns starting QB.

We spoke with CC owner Kelly Kandah who tells us it took 2 days to sculpt the double chocolate fudge cake ... which included edible gold bars and gold coins.

Final bill for the 45-pound monster ... $600!

Money.