Position rack in lower third of the oven and preheat to 375°F. Grease the bottom and sides of a 9-inch round springform pan and line with parchment.
Pulse pecans with flour in food processor until finely ground. Set aside. In large bowl, combine chopped chocolate, cocoa, and 3/4 cup sugar. Add boiling water. Whisk until chocolate is melted. Whisk in yolks and rum. Set aside.
Combine egg whites with cream of tartar in mixing bowl. Beat on medium until soft peaks form. Gradually sprinkle in rest of sugar; beat on high until stiff but not dry. Whisk flour and pecans into mixture. Fold about 1/4 of egg whites into chocolate mixture to lighten it. Fold in remaining egg whites. Scrape batter into prepared pan; level the top.
Bake 20-25 minutes or until toothpick comes out with a few moist crumbs clinging to it. Cool in pan, on a rack. The cake will sink in the center. Slide a small knife around inside of pan to detach cake. Invert cake; remove pan and paper liner. Turn right side up on platter, or leave cake on pan bottom and place on platter. To serve, sieve on powdered sugar; top with Classic Whipped Cream, if desired.