Preheat the oven to 400°F (200°C) and grease 4 ramekins with butter and dust with cocoa powder.
In a heatproof bowl, melt the dark chocolate and butter together in the microwave in 30-second intervals, stirring until smooth.
In a separate bowl, whisk the sugar and eggs until pale and fluffy. Slowly pour in the melted chocolate mixture and mix well.
Fold in the flour and salt until just combined. Do not overmix.
Divide the batter evenly among the prepared ramekins. Place the ramekins on a baking sheet and bake for 12-14 minutes until the edges are set but the center is still gooey.
While the cakes are baking, prepare the raspberry coulis. In a small saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the raspberries break down and the mixture thickens, about 5-7 minutes. Remove from heat and strain through a fine mesh sieve to remove the seeds.
Once the cakes are done, let them rest for 1-2 minutes before running a knife around the edges and inverting them onto serving plates.
Drizzle the warm raspberry coulis over the chocolate lava cakes and serve immediately.
Enjoy the decadent chocolate lava cakes with a burst of tangy raspberry flavor!