Preheat the oven to 350 degrees F.
Spread the pecans and walnuts on a small baking sheet and bake until fragrant and lightly toasted, about 8 minutes. Let cool.
Place the nuts and brown sugar in the bowl of a food processor and with the machine running, add the melted butter in a slow stream through the feed tube and process until combined. Reserve 2 tablespoons of the nut mixture for the topping and press the remaining mixture evenly across the bottom of a 10-inch springform pan.
Place the chocolate in the top of a double boiler or in a heatproof bowl set over barely simmering water and stir until melted. Set aside.
To make the meringue, combine the egg whites, confectioners' sugar, and vanilla in a large bowl and whip with an electric mixer until stiff peaks form. Set aside.
In a clean bowl using clean beaters, whip the cream with the coffee liqueur until stiff peaks form, being careful not to overbeat.
Place the chocolate in a large bowl and add 1/3 of the meringue, stirring until well combined. Fold in the remaining meringue in 2 additions, being careful not to overmix. Fold the whipped cream into the chocolate mixture, being careful not to deflate the mixture, and pour into the prepared pan. Sprinkle the reserved 2 tablespoons of the nut mixture over the top, wrap the pan with plastic wrap, and refrigerate overnight.
To serve, slice the cake using a thin, sharp knife dipped in warm water, and arrange on 12 plates. Drizzle the orange-caramel sauce to the side of each slice and serve.
Clear Orange-Caramel Sauce:
1 1/2 cups sugar
1/2 cup water
1 tablespoon grated orange zest
1 tablespoon fresh lemon juice
3/4 cup fresh orange juice
Combine the sugar, water, orange zest, and lemon juice in a medium heavy saucepan and cook over medium-high heat, stirring constantly, until the sugar dissolves. Once the sugar dissolves, stop stirring and continue to cook until the mixture thickens and turns golden brown, 10 to 15 minutes.
Remove from the heat and add the orange juice (the mixture will bubble up). Return to medium-high heat and cook, stirring constantly, for 1 minute. Remove from the heat and let cool slightly. Strain through a fine mesh strainer into a bowl and cool to room temperature before serving. (The sauce will thicken as it cools.)
Yield: 1 3/4 cups