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.