Heat the oven to 425 degrees F. Prepare a jumbo-sized muffin tin, coating the cups with non-stick cooking spray. Sprinkle with cocoa powder until well-coated. Tap off any excess.
Place the butter in a microwave safe bowl and cover with the chopped chocolate. Microwave on high in 20 second intervals, stirring between, until smooth. Be careful not to overcook.
Whisk together the flour, sugar, and salt. In a separate bowl, lightly beat the large eggs and egg yolks together. Add both the dry ingredients and the beaten eggs to the bowl of melted chocolate. Fold together with a rubber spatula until combined. Whisk out any lumps.
Spoon about half a cup of chocolate batter into each jumbo muffin cup (6 total). Make sure to leave enough batter to cover the cakes. Spoon just half a teaspoon of mixed berry jam into the center of each cup, and top with the remaining batter.
Bake for 10-15 minutes, checking every two minutes after the 10-minute mark. You’ll know it’s done when the sides are firm but the very center still looks uncooked. Remove from the oven and allow to cool for 1 minute.
Use a butter knife to gently release the sides of the cakes from the muffin tin. Place a large cutting board or baking sheet over the top of the cakes, and carefully flip the pan over. If all the cakes don’t drop out, you can gently pry them with the knife or a spoon.
Serve each cake on their own dish immediately with your topping of choice – whipped cream, shaved chocolate, and/or a scoop of ice cream.