Three ingredients—chocolate, eggs, and butter—meld together to create a smooth, rich, and chocolatey tart that will have your guests fainting in their chairs. It’s so simple and clever, and even more importantly for some, it contains no flour or added sugar beyond what’s in the chocolate. Dark chocolate is a must, and do splurge on European butter, which has more fat and flavor. Eggs whip better when they aren’t cold, so either temper them a bit over the double boiler after melting the chocolate or leave them on the counter for at least half an hour so they lose the chill from the fridge. To serve, decorate as you please, with a dusting of sugar, fruit, flowers, and even candles. I like it served cold, but it’s also delicious warm, with a scoop of ice cream or a dollop of whipped cream. Serves 10 10 1/2 ounces dark chocolate (70 …