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.
6. Spoon the batter into the cake pan or the prepared muffin tin, filling each cup half full. Bake for 18-20 minutes in a cake pan or 11-12 minutes in a muffin tin. The edges will have just begun to pull away from the pan, and the center should still look moist and shiny. Remove the pan from the oven and let cool for 10 minutes.