Make the crust: In a medium bowl, use a fork to combine the graham cracker crumbs and the butter until they are the texture of wet sand.
2. Step
Transfer the crust to an 8-inch (20 cm) springform pan and use the bottom of a wine glass or other flat instrument to press against the bottom.
3. Step
Make the dark chocolate mousse: In a medium bowl, let the melted dark chocolate chips cool to room temperature, then use a spatula to gently fold in whipped cream.
4. Step
Pour the dark chocolate mousse over the crust and smooth out the top as much as possible, then freeze for 15 minutes.
5. Step
Make the milk chocolate mousse: In a medium bowl, let the melted milk chocolate chips cool to room temperature, then use a spatula to gently fold in whipped cream.
6. Step
Pour the milk chocolate mousse over the dark chocolate layer and smooth the top with a spatula.
7. Step
Freeze for 2 hours, or until the mousse is set.
8. Step
Take a vegetable peeler and press it down along the edge of the chocolate bar to create mini chocolate spirals. Make at least 2 tablespoons worth.
9. Step
Remove from the cake from the freezer and gently take the cake out of the springform pan.
10. Step
Sift the cocoa powder over the top of the cake, then sprinkle with the chocolate shavings.
