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250g bittersweet chocolate, preferably Valrhona Guanaja
250g unsalted butter, at room temperature
4 large eggs at room temperature
70g all-purpose flour
Center a rack in the oven and preheat the oven to 180oC (350oC). Butter a 24-cm (9-inch) round cake pan that is at least 5 cm (2 inches) high, line the bottom with parchment paper, butter the paper, and dust the inside of the pan with flour; tap out the excess and set the pan aside.
Place the chocolate in a heatproof bowl over – not touching – simmering water and heat until the chocolate is melted; or melt the chocolate in a microwave oven. Set the chocolate aside to cool; it should feel only just warm to the touch when you mix it with the rest of the ingredients.
Put the butter and sugar in the bowl of a mixer and beat on medium speed for about 4 minutes, scraping down the sides of the bowl frequently, until the butter is creamy and the sugar well blended into it. Add the eggs one at a time, beating for about 1 minute after each addition. Reduce the mixer speed to low, pour in the cooled chocolate, and mix only until it disappears into the batter. Alternatively, you can fold in the last of the flour with a rubber spatula. You’ll have a thick, smooth, satiny batter that looks like old-fashioned chocolate frosting.
Scrape the batter into the pan, smooth the top, and slide the pan into the oven. Bake for 26 to 29 minutes or until the cake rises slightly and the top has lost its sheen. The top may crack a bit and the cake may not look entirely set in the center; when you test the cake by inserting a slender knife into the center, the knife will come out lightly streaked with batter, which is what you want. Transfer the cake to a rack to cool.
When the cake has cooled, chill it in the refrigerator for an hour or two to make it easy to unmould. Turn the cake out, remove the parchment, and invert the cake onto a serving platter so that it is right side up. Allow the cake to come to room temperature before slicing and serving.