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There are basically two types of flourless chocolate cakes. One is super rich and dense, like a slice of thick, rich ganache. The other is light and mousse-like thanks to the many whipped eggs which are folded into the chocolate. As this cake bakes, it rises, or "souffles", forming a crackly top crust that comes crashing down as the cake cools. Underneath, the texture spans from a cakey edge to a soft and fudgey center.
It can be hard to tell when this second type of cake is done because even though it rises, it still jiggles quite a lot and a toothpick inserted into the center always comes out wet. In the past, I've always overbaked this style of flourless cake out of fear that it hasn't set—and I'm invariably rewarded with a bone dry cake that crumbles away as I slice it. The trick is to use a thermometer, which I learned during my stint at America's Test Kitchen. (Hey, probing isn't just for meat.) Taking the temperature of the middle of the cake takes the guesswork out of the equation.
So why call it a baked chocolate mousse cake? Because not only is the batter just that (melted chocolate, butter, whipped eggs), but the texture of this cake when served at room temperature is so creamy, luscious, and moist, it's just like a big slice of decadent chocolate mousse. For firmer, denser texture, simply serve the cake directly out of the fridge.