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 rack and cool.
FILLING
In a medium heavy saucepan, bring cream to a simmer. Add chocolate and stir until melted and smooth.
Pour chocolate mixture into crust. Cover and refrigerate until firm (about 2 hours).
SAUCE
Melt butter in heavy medium skillet over medium-high heat. Add sugar and cook until deep golden brown, whisking frequently, about 10 minutes. (The mixture will be grainy.)
Add cream and boil until sauce is reduced to 2½ cups, whisking occasionally (about 10 minutes). Cool slightly.
When ready to serve, remove tart bottom. Cut tart into wedges and transfer to plates. Spoon warm sauce around 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.