Blend first 3 ingredients in food processor until nuts are finely ground. Add butter and blend until well-combined. Press nut mixture onto bottom and sides of 9 inch tart pan with a removable bottom.
Bake until golden brown, about 20 minutes. Transfer to a rack and cool.
MAKE THE FILLING:
In a medium heavy saucepan, bring the cream to a simmer. Add the chocolate and stir until melted and smooth.
Pour the chocolate mixture into the crust. Cover and refrigerate until firm (about 2 hours).
PREPARE THE SAUCE:
Melt the butter in heavy medium skillet over medium-high heat. Add the sugar and cook until deep golden brown, whisking frequently, about 10 minutes. (The mixture will be grainy.)
Add the cream and boil until sauce is reduced to 2½ cups, whisking occasionally (about 10 minutes). Cool slightly.
When ready to serve, remove the tart bottom. Cut the tart into wedges and transfer the wedges to plates. Spoon the warm sauce over and around the tart wedges.
Notes
If you are preparing this dessert as gluten free, just be sure to use brands of chocolate that are known to be GF.
To save time on the day of entertaining, you may prepare the crust/filling 2 days ahead).
You may also prepare the sauce 2 days ahead. Cover and refrigerate. Before serving, rewarm sauce over medium heat, stirring frequently.