To make the crust: In a food processor, combine the walnuts, pecans, brown sugar, and cinnamon and pulse until the nuts are coarsely chopped.

Pour the nuts into a bowl, add the butter, and stir until the nuts are evenly moistened. Press the nut mixture evenly onto the bottom of a 9-inch round springform pan. Refrigerate for 30 minutes.

To make the filling: Melt the chocolate in a double boiler. While the chocolate is melting, combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until smooth, about 1 minute. Switch to the whip attachment and add the eggs two at a time, mixing well after each addition. Scrape down the sides of the bowl, increase the speed to medium-high, and whip until well blended, about 2 minutes. The mixture will look curdled.

Remove the melted chocolate from the heat and stir until smooth and warm but not hot. With the mixer on low speed, add the chocolate to the egg mixture. Scrape the sides and along the bottom of the bowl and then continue to mix on low speed until the chocolate is frilly incorporated. Add the cream and vanilla and mix until blended. Spread the filling over the crust. Refrigerate until firm, about 3 hours.

To make the topping: In a bowl, whisk together the cream and sugar until soft peaks form. Cover and refrigerate until serving.

Using a spatula, spread the topping over the top of the cake. Run a hot, dry knife around the inside edge of the pan to loosen the cake, then remove the pan sides and set the cake on a serving platter. Slice the cake with a hot, dry knife and serve chilled.

Planning ahead:
The cake may be made 2 days in advance and kept in the refrigerator. The topping can be made up to 4 hours in advance, covered, and refrigerated. It may need to be lightly rewhipped before using. You may put the topping on the cake up to 2 hours before serving and keep the cake in the refrigerator. If storing foods with strong odors, such as shrimp, invert a large bowl over the cake to protect it from them.