Place oven rack in center position and heat oven to 350 degrees F. Spread nuts on shallow baking sheet, toast in oven until deep golden brown, 10 to 12 minutes. Cool completely. Transfer nuts to food processor or blender, add flour and salt, and grind mixture fine. Set aside.

Increase oven temperature to 375 degrees F. Generously butter bottom and side of an 9-inch springform or cake pan, line bottom with a round of parchment paper, butter parchment paper. Dust pan with cocoa powder, knocking out excess.

Melt chocolate and butter in double-boiler or stainless steel bowl set over a saucepan of simmering water. Set aside.

Combine egg yolks and 1/2 cup sugar in a large mixing bowl. Beat, using an electric mixer or by hand, until pale and thick, about 4 to 5 minutes (6 to 8 minutes by hand). Beat in vanilla and liqueur, if desired. Gently fold in chocolate and nut mixtures. Set aside.

In a medium bowl, whisk egg whites, using mixer with whisk attachment or by hand, until foamy. Add cream of tartar, whisking until soft peaks form. Gradually sprinkle in remaining 1/4 cup sugar, whisking until whites just hold stiff peaks. Whisk about one-fourth of the egg whites into chocolate mixture, to lighten. Using a large spatula, gently but thoroughly, fold in remaining egg whites. Be careful not to over mix. Pour batter into prepared pan and smooth top. Bake until tester or toothpick inserted in center of torte comes out with a few moist crumbs clinging to it, about 35 to 45 minutes.

Let torte cool completely in pan on a wire rack, about 1 hour. (Torte will sink in the center as it cools leaving a raised crust around the edge along with some cracking. Level top of torte by lightly pressing edges and any cracked pieces back into the cake as it cools) When torte has completely cooled, run a thin knife around edge of torte to loosen, as necessary. Remove sides of spring form and/or invert pan to unmold. Remove parchment liner from bottom and turn torte upright on a corrugated cake circle or platter.

For advance preparation, torte may be prepared to this point, covered, and stored at room temperature or in the refrigerator for 1 to 2 days; or frozen, well wrapped in plastic and foil, for up to 2 months.

To serve: Dust torte with powdered sugar and serve, or, if desired, coat torte with ganache.

To make Ganache: Place chopped chocolate in a medium stainless steel bowl; set aside. Heat cream and butter in a medium saucepan over medium heat until just reaching a boil. Immediately pour hot cream over chocolate, let stand 5 minutes. Whisk until smooth. If necessary, place bowl over a saucepan of simmering water to melt chocolate completely. Whisk in liqueur, if desired. Let stand until slightly thickened, about 10 minutes. Ganache is ready to use when it mounds slightly when dropped from a spoon. It will become firmer the longer it cools.

Place torte on wire rack placed over a baking sheet, brush away any loose crumbs. Lightly cover sides and top of torte with 2 to 3 tablespoons of ganache, to make a crumb coat. Refrigerate 5 to 10 minutes to set crumb coat. Pour remaining ganache onto top center of torte. Quickly spread with spatula, using big strokes to push ganache over sides of cake. Cover any bare spots on sides of torte with leftover ganache from baking sheet (*see note). If desired, garnish top and/or side of torte with chopped toasted pecans. Let torte stand until glaze is set, about 2 hours, or cover and refrigerate about 1 hour. Serve torte with whipped cream or ice cream, if desired.

Makes one 9-inch cake, or about 10 to 12 servings

* Note: Any leftover ganache can strained, to remove crumbs, and saved for use later.