1. Place a rack in the middle of the oven and preheat to 350 F.
2. Grease or spray a 9-inch cake pan or 12 cup muffin tin with vegetable or canola oil. Wipe out the excess oil with a paper towel. Set aside.
3. Grate the chocolate or chop it into small shards with a chef’s knife or a serrated knife. Place the chocolate in the top of a double boiler or a large bowl set over a saucepan of simmering water over low heat. Melt the chocolate slowly, stirring occasionally.
4. In a medium bowl, gently whisk together the eggs, egg yolks, sugar, and flour. Mix until all the ingredients are well incorporated.
5. When the chocolate is smooth, remove the bowl from the hot water and add the butter. Stir until the butter is melted and the chocolate is smooth again. Add the egg mixture, mixing well.