Preheat the oven to 325°F (160°C). Grease a 9-inch (23cm) round cake pan and line the bottom with parchment paper.
In a heatproof bowl, combine the chopped dark chocolate and butter. Place the bowl over a pot of simmering water, making sure the bottom of the bowl doesn’t touch the water. Stir occasionally until the chocolate and butter have melted and the mixture is smooth. Remove from heat and let it cool slightly.
In a separate mixing bowl, whisk together the sugar, eggs, vanilla extract, and salt until well combined.
Slowly pour the melted chocolate mixture into the egg mixture while whisking continuously. Mix until the batter is smooth and glossy.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for 35-40 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. The cake will be set but still slightly jiggly in the center.
Remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, carefully transfer it to a wire rack to cool completely.
While the cake is cooling, 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 slightly, about 5-7 minutes.
Remove the saucepan from the heat and let the coulis cool. Once cooled, strain the mixture through a fine-mesh sieve to remove the seeds. Press down with the back of a spoon to extract as much liquid as possible.
To serve, slice the flourless chocolate cake into wedges and drizzle each slice with raspberry coulis. Garnish with fresh raspberries and a dusting of powdered sugar, if desired.
Enjoy the decadent flourless chocolate cake with raspberry coulis immediately or refrigerate any leftovers in an airtight container for up to 3 days.