Preheat the oven to 355°F (180°C). Prepare an 8-inch (20cm) springform pan or a cake ring wrapped with heavy-duty aluminum foil. With the softened butter and a pastry brush, grease the bottom and the sides of the mold. Line them with strips and a disk of parchment paper. Place the prepared cake ring on a baking sheet.

Melt chopped chocolate with butter cut into small cubes in the microwave at 30-second increments or in a double boiler, occasionally stirring with a wooden spoon, and set aside to cool down. 

Alternatively, use a water bath: place chopped chocolate with butter in a Pyrex glass bowl (or another heatproof bowl) over a small saucepan with some water. Bring the pan over medium-high heat and let the ingredients melt, occasionally stirring with a wooden spoon.

In the bowl of a stand mixer or a separate mixing bowl, place sugar and eggs (at room temperature) and whisk for 5 minutes until the mixture doubles in volume. You can use an eclectic mixer if desired. Using a rubber spatula, gently fold the sifted flour into the egg and sugar mixture. Gently incorporate the cooled melted chocolate into the batter with a spatula, taking care not to deflate it.

Pour the chocolate batter into the prepared mold and bake for 35 minutes. Remove the cake from the oven and let it cool before unmolding.