Preheat oven to 375 degrees F. Butter an 8-inch springform pan and place a round of parchment paper on the bottom and butter the paper.
For the cake, combine the chocolate and butter in a small saucepan. Melt over low heat, stirring constantly. Remove from the heat and stir in sugar. Let cool slightly.
Whisk in the eggs, one at a time, combining thoroughly between additions. Whisk in the vanilla and sift the cocoa powder into the mixture, whisking to combine. Pour into the prepared pan and bake in the center of the oven for 25 minutes or until the top has set.
Cool in pan for 5 minutes, then run a knife around the edge and gently turn onto a serving platter. Gently remove the parchment. Let cool for at least an hour before serving. Dust with cocoa powder if desired.
For the coulis, combine the sugar, water, and raspberries in a medium saucepan over medium heat, stirring frequently, for 5 minutes or until all of the sugar has dissolved.
Puree the berry mixture and lemon juice in a blender until smooth. Strain through a fine-mesh strainer to remove the seeds.
To serve, drizzle the plate with raspberry coulis and place a slice of cake on top. Serve with fresh raspberries and a dollop of fresh whipped cream (if desired).
MAKE AHEAD: The cake can be made 1 to 2 days ahead and kept, tightly wrapped, at room temperature. The sauce can be kept in a sealed container in the refrigerator for 4-5 days.