Alas I don't have one to share, but was thinking about this just now (inspired by a pic I saw of someone with tiles on a cookie sheet)... a full edible HeroQuest would be a lot of work (and then somebody nibbles your chocolate barbarian), but what about a birthday cake?
It would be one of those big sheet cakes, and you could decorate it with a grid to resemble the board. You could even toss some miniatures (not painted!) on top for decoration (or re-cast them in wax and like the swords could be candles or something). Maybe there could be a sorcerer's table with ACTUAL candles for the cake?
I wouldn't recommend putting cardboard into the frosting, but little door or furniture replacements made of candy or something would be cool. Maybe they could be "carved" out of those wafer sheets you see in the baking aisle? Or pieces of sandwich cookies? licorice?
Or maybe you could "make" minis out of fun fruits (fruit wrinkles, whatever those are called... fruit snacks) of a certain color. Red for heroes, green for greenskins, white (yogurt covered raisins?) for undead. To simulate a game in progress on top of the cake. Tiles could be simulated with gel frosting or whatever. Maybe marshmallows for dice (do food coloring pens exist?), or white chocolate cut into cubes (and some with red food coloring for the movement dice).
You could get really creative with it. Just throwing that idea out there.
They also have those "screen printed" (on "sugar paper") Birthday cakes, so you could even probably screen print the image of the board onto the cake itself instead of having to use a ton of gel icing to make the board. Of course you could just go easy and have it be the Les Edwards cover or the "Mentor/Zargon" GM screen image.