Adapted from this recipe for classic chocolate Texas sheet cake from the food network.
Store any leftover cake in the fridge. It will keep fresh for days. It also freezes very well.
A few notes on the frosting: Traditional Texas sheet cake frosting is more solid and hard since it relies on the mixture of sugar and butter. I felt like going with a slightly more soft frosting {since the cake is in a pan that's very forgiving and I'll take a good gooey melted chocolate topping over a crunchy one any day ;) }.
This particular frosting has a solid soft spreadable consistency after chilling. If however you frost your cake while it's still warm/hot, then the frosting will turn slightly more gooey {as you can see in the pictures of the finished cake, which I frosted while still warm}. I love it that way, but if you prefer a more solid frosting, then make sure your cake is chilled first.
Either way, the frosting will solidify once the cake has chilled in the fridge.
For a more traditional frosting, have a look at the frosting in this recipe {but use non-dairy butter and milk}.