Preheat the oven to 425°F (220°C) and grease four 6-ounce ramekins with butter. Dust with flour and set aside.
In a heatproof bowl, melt the butter and chocolate together in the microwave or over a double boiler. Stir until smooth and set aside to cool slightly.
In a separate bowl, whisk the eggs, egg yolks, and granulated sugar until pale and fluffy.
Slowly pour the chocolate mixture into the egg mixture, whisking constantly to combine. Stir in the vanilla extract and salt.
Sift the flour over the chocolate mixture and gently fold 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 soft.
While the cakes are baking, prepare the raspberry coulis. In a small saucepan, combine the raspberries, granulated 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 seeds. Set aside to cool.
Once the cakes are done, remove them from the oven and let them rest for 1 minute. Carefully invert each cake onto a plate and dust with powdered sugar.
Drizzle the warm raspberry coulis around the cakes and serve immediately. Enjoy the molten chocolate goodness with a burst of tangy raspberry flavor!