Pulse chocolate wafers in a food processor until you have fine crumbs, or place in a zip-top bag and crush with a rolling pin. Stir in melted butter until the mixture looks like wet sand. Press firmly into the bottom of a 9-inch springform pan, using the bottom of a measuring cup to create an even layer. Chill while you prepare the fillings.

Combine 8 oz of the raspberries with 2 tablespoons sugar in a small saucepan over medium heat. Cook, mashing with a fork, until the berries break down and become saucy, about 5 minutes. Press through a fine-mesh sieve into a bowl, discarding the seeds. You should have about 1/2 cup smooth puree.

Sprinkle gelatin over cold water in a small bowl and let bloom for 5 minutes. Stir the gelatin mixture into the warm raspberry puree until completely dissolved. Let cool to room temperature, about 15 minutes. The mixture should still be pourable but not warm to the touch.

Pour the raspberry mixture over the chilled crust, spreading to the edges. Reserve the remaining 4 oz whole raspberries for garnish. Refrigerate until set, about 30 minutes. The surface should feel firm to the touch before adding the chocolate layer.

Place chopped chocolate in a medium bowl. Heat 3/4 cup of the cream in a small saucepan until steaming and small bubbles form around the edges. Pour over the chocolate and let stand 2 minutes, then stir until completely smooth. If any chocolate pieces remain, microwave in 15-second bursts, stirring between each, until fully melted.

In a large bowl, whisk egg yolks and 2 tablespoons sugar until pale and thickened, about 2 minutes. Slowly whisk in the warm chocolate mixture until fully combined. The mixture should be glossy and smooth.

Beat remaining 1 1/4 cups cream to medium peaks - the cream should hold a soft peak that folds over slightly at the tip. Gently fold about one-third of the whipped cream into the chocolate mixture to lighten it, then fold in the remaining cream in two additions. Stop when no white streaks remain; the mousse should be airy but still pourable.

Pour the chocolate mousse over the set raspberry layer, spreading to the edges with an offset spatula. Smooth the top, then refrigerate uncovered for at least 4 hours, preferably overnight, until completely set. The mousse should feel firm and spring back slightly when pressed.

Run a thin knife around the edge of the pan before releasing the springform. Dust the top with cocoa powder through a fine sieve. Arrange reserved fresh raspberries over the top. For clean slices, dip a sharp knife in hot water and wipe dry between each cut. Serve chilled.