Step 1: Preheat your oven to 425°F (220°C). Grease a 6-cup muffin tin with butter or line it with cupcake liners.
Step 2: In a microwave-safe bowl, combine the butter and chocolate chips. Microwave in 30-second intervals, stirring after each, until the mixture is completely melted and smooth. Set aside to cool slightly.
Step 3: In a separate medium bowl, whisk together the whole eggs, egg yolks, granulated sugar, and vanilla extract until well combined and slightly pale.
Step 4: Gradually pour the slightly cooled melted chocolate mixture into the egg mixture, stirring constantly until fully incorporated. Be sure to stir well to combine without scrambling the eggs.
Step 5: Gently fold in the all-purpose flour and salt until just combined. Be careful not to overmix the batter.
Step 6: Spoon about 1 tablespoon of batter into the bottom of each prepared muffin cup. Place 3-4 fresh raspberries on top of the batter in each cup. Cover the raspberries with another tablespoon of batter, ensuring they are mostly submerged and hidden.
Step 7: Place the muffin tin in the preheated oven and bake for 12-14 minutes, or until the edges of the cupcakes are set but the centers still appear soft and slightly jiggly. The tops should look slightly puffed and possibly cracked.
Step 8: While cupcakes are baking (optional Raspberry Sauce): In a small saucepan, combine 1 cup fresh raspberries, 2 tablespoons sugar, and 1 tablespoon lemon juice. Cook over medium heat, stirring gently, until the raspberries break down and the sauce thickens slightly, about 5-7 minutes. Strain through a fine-mesh sieve if you prefer a seedless sauce.
Step 9: Remove the cupcakes from the oven and let them cool in the muffin tin for 1-2 minutes. Carefully invert each cupcake onto a serving plate. Serve warm, drizzled with the optional raspberry sauce and garnished with extra fresh raspberries. Enjoy immediately for the best gooey center experience!