Line a 9-inch springform pan with a round of parchment paper; set aside.

In the bowl of a food processor, combine the pecans, walnuts, and brown sugar and pulse until the nuts are coarsely chopped. Pour the nut mixture into a medium bowl, add the butter and salt, and stir until the nuts are evenly moistened. Press the nut mixture evenly into the bottom of the prepared pan. Refrigerate for 30 minutes or until set.

Melt the chocolate and espresso in a bowl set over a pan of simmering water. 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, add the chocolate to the egg mixture. Scrape down the sides and bottom of the bowl and mix on low speed until the chocolate is fully incorporated. Add the cream and vanilla and mix until blended. Spread the filling over the crust and refrigerate until firm, about 3 hours. (The torte can be made and refrigerated 2 days in advance.)