Step 1: Prepare the Raspberry Filling

1. In a small saucepan over medium heat, combine the raspberries, sugar, and lemon juice.
2. Stir and mash the raspberries with a fork until they release their juices. Let the mixture simmer for about 5 minutes.
3. If using cornstarch, mix it with a teaspoon of water and stir it into the raspberry mixture to thicken.
4. Remove from heat and let it cool completely.

Step 2: Make the Chocolate Batter

1. Preheat your oven to 425°F (218°C). Grease 6 muffin tins or ramekins with butter and dust lightly with cocoa powder.
2. In a heatproof bowl, melt the butter and chopped chocolate together over a double boiler or in the microwave (in 20-second increments, stirring between each).
3. Once melted and smooth, remove from heat and whisk in the sugar.
4. Add the eggs and egg yolks, one at a time, whisking until fully incorporated.
5. Stir in the vanilla extract and salt.
6. Sift in the flour and cocoa powder, then gently fold the mixture together until smooth.

Step 3: Assemble the Lava Cupcakes

1. Fill each prepared muffin tin halfway with chocolate batter.
2. Add a teaspoon of raspberry filling to the center of each, then top with the remaining chocolate batter, ensuring the filling is fully covered.
3. Bake for 10–12 minutes, or until the edges are set but the centers are still slightly wobbly.

Step 4: Serve Warm and Enjoy!

1. Let the cupcakes rest for 1–2 minutes before carefully running a knife around the edges and inverting them onto plates.
2. Dust with powdered sugar, add fresh raspberries, or serve with vanilla ice cream for an extra treat!