Prepare the raspberry lava center: Combine raspberries, sugar, and lemon juice in a saucepan. Cook until raspberries break down and mixture thickens. Stir in chopped chocolate until melted. Set aside.
Preheat oven to 425°F (220°C). Grease and flour a 6-cup muffin tin.
Melt butter and chocolate together until smooth.
Whisk sugar, eggs, and egg yolks until light and thickened.
Whisk the melted chocolate mixture into the egg mixture. Fold in flour and salt until just combined.
Spoon half of the chocolate batter into each muffin cup. Spoon raspberry lava center into the middle. Top with remaining batter.
Bake for 12-15 minutes, or until edges are set but center is soft.
Cool for a few minutes before inverting onto a wire rack. Serve warm.
Notes
Serve warm with fresh raspberries and a dusting of powdered sugar (optional).