Preheat the oven to 375 degrees Fahrenheit, and butter an 8-inch round cake pan. Line the base of the pan with parchment, and butter the parchment too. Chill in fridge.
Finely chop the chocolate (a serrated bread knife does an outstanding job of this) and melt it gently with the butter in a double boiler or in the microwave, stirring regularly to combine. Add the sugar to the chocolate-butter mixture, stirring well, and set aside to cool for a few moments. Then add the eggs one by one, stirring well after each addition, and then add the flour. The batter should be smooth, dark, and utterly gorgeous. Chill before baking.
Pour batter into the buttered cake pan and bake for approximately 25 minutes, or until the center of the cake looks set and the top is shiny and a bit crackly-looking. (I usually set the timer for 20 minutes initially, and then I check the cake every two minutes thereafter until it’s done. At 20 minutes, it’s usually quite jiggly in the center. You’ll know it’s done when it jiggles only slightly, if at all.) Let the cake cool in its pan on a rack for 10 minutes; then carefully turn the cake out of the pan and revert it, so that the crackly side is facing upward. Allow to cool completely. The cake will deflate slightly as it cools.
Serve in wedges at room temperature with a loose dollop of ever-so-slightly sweetened whipped cream.
Note: This cake is even better on the second day, so consider making it the day before serving. And thank you to Clotilde of Chocolate & Zucchini, whose mention of a clementine cake in Trish Deseine’s Mes petits plats préférés led to my stumbling upon Je veux du chocolat! in the bookstore one day.