Chocolate Magic Custard Cake Recipe

"Recipe is adapted from our previously posted Magic Custard Cake. A few notes on making the cake: the vinegar helps stabilize the egg whites after they've whipped to stiff peaks. If you have a copper bowl to whip them in, you can skip the vinegar. When adding the milk, we found it easier and less messy to gently hand whip them in instead of using the stand mixer. It is a very liquidy batter, and for us it splattered everywhere even at the slowest speed. You could also wrap the head of the mixer and the top of the bowl with plastic wrap to help contain the splattering. As with the original magic custard cake, the temperatures of the ingredients are important. If too hot it will cook the eggs, too cold it will harden the butter. For folding in the whites, it is done with more of a gentle whisk/folding motion with the whisk since the batter is so runny. The whites will look a little curd like after mixing them in, just try to mix/fold them to the point where there is no big chunks. Because of the custard center, when fully baked, there will be a bit of a jiggle, but not a sloppy jiggle to the cake when gently shook. Same after it has cooled. That's about it. Have fun and every time we've made the magic custard cakes (this one or the original), going into the oven we don't think they will work correctly, and after baking they are magically perfect...."