How to Make It
Preheat the oven to 375°. Butter a 9-inch round springform pan. Line the bottom of the pan with parchment paper and butter the paper. Spread the pecans on a large rimmed baking sheet and bake for 8 minutes, or until lightly toasted. Transfer to a plate and let cool, then coarsely chop. Leave the oven on.
In a large saucepan, bring 2 inches of water to a simmer over low heat. In a large stainless steel bowl, whisk the eggs with the granulated and confectioners' sugars. Set the bowl over the simmering water and whisk the egg mixture constantly until it is warm to the touch, about 3 minutes. Remove the bowl from the heat.
Put the butter and chocolate in a medium stainless steel bowl and set over the simmering water until just melted, about 3 minutes. Add the vanilla, stir well and remove from the heat.
Using a handheld electric mixer, beat the egg mixture at high speed until pale yellow and doubled in volume, about 5 minutes. Add the flour all at once and gently fold it in with a whisk. Using a rubber spatula, fold in the melted chocolate and the pecans. Pour the batter into the prepared pan and set it on a baking sheet. Bake for 20 minutes. Reduce the oven temperature to 350° and continue to bake the cake for about 50 minutes longer, or until set and an instant-read thermometer inserted in the center registers 200°.
Transfer the cake to a wire rack and let cool to room temperature in the pan, about 4 hours. Remove the side of the springform pan and pierce any bubbles that formed on the surface of the cake.
In a small saucepan, bring the cream to a boil. Put the chocolate in a medium bowl, pour the hot cream over it and let stand for 5 minutes, or until the chocolate is melted. Add the Grand Marnier and whisk until smooth. Set aside, whisking occasionally, until shiny and just warm to the touch, 1 1/2 to 2 hours.
Invert the cake onto a plate and peel off the parchment paper. Turn the cake over onto a rack set over a baking sheet. Pour 1/3 cup of the ganache over the top of the cake and spread it evenly with an offset spatula. Refrigerate the cake on the rack until set, about 20 minutes. Slowly pour the remaining ganache over the cake, spreading it over the top and around the sides. Refrigerate the cake until the ganache is set, about 20 minutes. Cut the cake into wedges with a hot knife, wiping the blade between cuts.