Preheat your oven to 325°F (160°C). Grease and flour a 9-inch springform pan. You can also line the bottom with parchment paper for easier removal. This ensures your Flourless Chocolate Cake releases cleanly after baking.
In a heatproof bowl set over a saucepan of simmering water (or in the microwave in 30-second intervals), melt the chocolate and butter together, stirring until smooth. Be careful not to burn the chocolate. Once melted, remove from heat.
In a separate bowl, whisk together the sugar, cocoa powder, and salt. This will help to evenly distribute the cocoa and prevent any lumps in your cake batter.
Pour the melted chocolate mixture into the sugar mixture and stir until well combined. The batter will be thick and glossy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract. Make sure each egg is fully incorporated to create a smooth and even batter for the Flourless Chocolate Cake.
Pour the batter into the prepared springform pan and spread evenly. Bake for 30-35 minutes, or until the edges are set but the center is still slightly soft. A toothpick inserted into the center should come out with moist crumbs attached.
Let the cake cool in the pan for 15-20 minutes before releasing the sides of the springform pan. Allow it to cool completely on a wire rack before serving. This prevents the Flourless Chocolate Cake from breaking apart.
While the cake is cooling, make the raspberry coulis. In a saucepan, combine the raspberries, sugar, and lemon juice. Bring to a simmer over medium heat and cook for 5-7 minutes, or until the raspberries have softened and the sauce has thickened slightly.
Press the raspberry mixture through a fine-mesh sieve to remove the seeds. This will give you a smooth and velvety coulis to drizzle over your Flourless Chocolate Cake.