1.	Position a rack in the middle of the oven and preheat the oven to 350°F. Lightly butter an 11-inch fluted tart pan with a removable bottom.
2.	To make the crust: Process the sugar and walnuts in a food processor until the walnuts are finely ground. Add the flour, cocoa, and salt and pulse just until blended. Add the butter and pulse just until the mixture begins to come together when a small amount is pressed between your fingers; do not overprocess—the mixture should not form a ball. Press the dough evenly into the bottom and up the sides of the tart pan. Prick the dough all over with a fork.
3.	Bake for 15 to 18 minutes, until the crust begins to pull away from the sides of the pan. Let cool on a wire rack while you make the filling.
4.	To make the filling: Melt the chocolate with the butter in a heatproof bowl set over a saucepan of about 1-1/2 inches of nearly simmering water, whisking until smooth. Remove the bowl from the heat and whisk in the eggs and sugar until well blended. Whisk in the vanilla. Transfer the filling to the warm crust.
5.	Bake for about 12 minutes, until the filling is set around the edges but still slightly jiggly in the center; the top of the tart will look a little blistered, and that’s okay. Transfer to a wire rack to cool completely, at least 1-1/2 hours.
6.	To serve, remove the pan rim. Lightly dust the tart with cocoa powder and cut into wedges.