You've got to love Minecraft for working a simple, square cake right into the game. It makes things so much easier for parents planning Minecraft parties. I noticed most of the folks posting pictures of their cakes online, favor fondant over butter cream frosting. While fondant does lend itself nicely to crisp lines, and a smooth finish - canned frosting (or homemade) is much simpler to work with, and a good deal less expensive - as well as being quite tasty. And, as far as the Minecraft cake goes, it's totally doable.
Simple start with a square cake - bake one or two cake mixes worth of batter in two square pans, allow to cool slightly, and then put them together with a layer of frosting. Freeze the cake, covered, overnight...
...to eliminate the need for a crumb coating of frosting. You might also want to slip strips of tinfoil under the bottom edges of the cake, to keep the plate clean, while you're frosting (both of those are old Martha Stewart tips).
Then, you can begin by piping the square pattern onto the sides of the cake, with vanilla frosting. You don't have to get too fancy. I used a resealable baggie with the corner snipped off for the piping pictured.
Continue piping to fill in the outline on the sides. Blop a good dollop of frosting out on top, and smooth it out with a butter knife.
If you are making a yellow cake, then that might be all the frosting you need. But, if your birthday child prefers chocolate cake, or you just really like frosting...
...fill in the bottom of the cake with a second piping bag of butter cream frosting.
Use kitchen shears to snip out a few fruit roll-up squares for the top of the cake. Remove the tinfoil strips, allow a few hours of cake thawing time...
...a candy filled Creeper, or two (a free paper craft found here).
It's great to be a homeschooler.