If you've ever baked at altitude you are probably a little sick of tinkering with recipes, and just a bit afraid every time you bake that your cake will sink. When you need to make a perfect cake at altitude and nothing less will do, I suggest making an ice cream cake. It's guaranteed to not lose it's shape until it melts.
Here's what you need:
waxed paper (optional)
9-inch cake pan
1/2 gallon ice cream
spatula or large flat spoon
6 ounces bittersweet chocolate
3/4 cup heavy cream
Set your ice cream out to soften, about 15-20 minutes. You can speed up the process a bit by mixing it in the bowl of a stand mixer, but still let it soften for a good ten minutes first.
Set out your cake pan and plastico. Roll out two 30-inch (apx) pieces of plastico and set one in the pan, pressing it into the bottom and sides and hanging off evenly. Set the other piece in the same fashion, only with the overlap hanging out perpendicular to the first piece (meaning, if the first piece is set longitudinally, set the second latitudinally). It should look like this:
When the ice cream is nice and soft, spoon it out and press it into the cake pan. Don't worry too much about pockets, just try to fill it evenly. When all the ice cream is in the pan, smooth off the top with an offset spatula or the best spready-tool you've got. Cover with waxed paper (or another piece of plastic wrap if you don't have waxed paper) and press down to pack in the ice cream and eliminate air pockets. If you have another cake pan you can use it to press down on the filled ice cream cake. Fold the overhanging plastic wrap up and over the top and stick the pan in the freezer to set, for at least 3 hours. I like to give it a full 24 hours.
Next, make the frosting.
I like ganache for ice cream cakes. Finely chop your chocolate and transfer it to a glass or stainless steel mixing bowl.
Heat the cream, watching it carefully as it makes a colossal mess if it boils over. You can add flavorings to the cream like vanilla extract (a teaspoon), or liquors (about a tablespoon).
Once the cream comes to a boil, pour it over the chocolate and stir gently until smooth. Cool for about 15-20 minutes.
Remove the ice cream cake from the freezer. Unwrap the plastic from the top. I often leave the waxed paper on as a base, but you run the risk of serving it, so it's maybe a smarter idea to remove that too. You want to work quickly, but don't get stressed - it's not going to melt all over the table on you. Invert the cake onto a serving plate or a work plate the plastic sticking out the sides. Wet a dishcloth or dishtowel with hot water and press it onto all sides of the cake. It should only take a moment before the ice cream cake releases itself from the pan. If not, keep the towel hot and pull gently on the plastic wrap. If the cake looks a little melty from the heat exposure, stick it back in the freezer for 30-45 minutes.
When ready to frost, scoop about three-quarters of the ganache onto the top of the cake and spread it evenly over the top of the cake, allowing it to flow down the sides. Use more ganache as needed to frost the sides. Using a thin coat of ganache will work better as it will set quickly and freeze onto the cake. When it's all covered and smooth (using an offset spatula for frosting will make spreading and smoothing much easier) return it to the freezer. Another trick for smoothing is to heat the spatula. Do this once the entire cake is frosted and it will give the cake a clean finish.