Place the oven rack in the center of your oven and preheat oven to 450 degrees.
Grease inside cavities of the mini heart pan or muffin tin: using a soft bristled pastry brush, lightly brush the inside of the pan thoroughly with vegetable oil.
Place a medium saucepan over low heat and add the chocolate and butter. Stir constantly until chocolate is melted. Remove from heat immediately. Allow to cool for a few minutes.
Add the eggs, egg yolk, powdered sugar, flour and salt. Mix vigorously with a french whisk until well combined. The batter should have no lumps.
Divide the batter equally into your pan. If using the heart pan, bake for 10-12 minutes, until the sides are fully set, and the center is puffy but still slightly soft. If using a muffin tin, bake for only 8-10 minutes. You can double check the doneness by inserting a toothpick into the sides of the cake - it should come out slightly wet.
Remove from oven. Loosen just the top edges of the cake with a butter knife or thin icing spatula and immediately invert cakes onto a cooling rack.
To serve, place on individual plates while still warm and sprinkle with powdered sugar. Top with ice cream, whipped cream, and/or berries as desired. These cakes are best served still warm, no more than 10 minutes after they've finished baking. If allowed to cool the insides will set.
