How do I make this cake? I'm unsure how to make a support system for this without the hat sliding off. Any detailed advice or online tutorial would be wonderful. Thanks :)

You could use a styrofoam wedge with melted chocolate to keep it on and a center dowel so it doesn't slide...all hidden with the legos.

AI'm thinking that the top is done similar to a Topsy-Turvy. Just behind that wall of Legos is the actual cake that goes down to the base. It's just a cantilevered illusion. Just enough meat removed for the Lego pieces to rest in, giving the illusion of a slanted top. I could be wrong. I usually am.

Here is a pattern to make a treasure chest.  It is based on the same idea.  #2 is a wedge of cake that holds the top.   You can then dowel all the way through #3, 2 and 1 and it should hold the top.  The additional Leggos in front should also hold the part of #3 which looks like it is floating.  I would buy real Leggos to put in between the cakes.  I am not hung up on everything edible and it would be a more sturdy option to lay the top on (#3).