Preheat the oven to 350 degrees . Line the bottom of a greased 10-inch springform or regular cake pan with parchment paper and grease the paper.
In a small skillet, melt the butter over low heat. Scrape the vanilla seeds into the butter, add the vanilla bean and simmer until the butter is lightly browned, about 5 minutes. Discard the vanilla bean and let cool.
Using a food processor, grind the nuts with the confectioners' sugar and flour. Add the orange peel and pulse until just blended.
Using an electric mixer with the whisk attachment, whip the egg whites and 1/4 teaspoon salt at high speed until frothy. Reduce the speed to medium, and gradually add 4 tablespoons granulated sugar. Whip on high speed until stiff and glossy. By hand, fold half of the nut mixture into the egg whites until nearly blended. Add half of the cooled butter and fold until just combined. Repeat with the remaining flour mixture and cooled butter.
Scrape the batter into the prepared pan. Bake until the cake is springy to the touch and golden brown, 35 to 40 minutes. Transfer the cake to a rack to cool, about 20 minutes. Invert the cake and remove the pan and parchment paper. Let the cake cool right side up, about 35 minutes.
Meanwhile, in a large heatproof bowl, mix together 5 tablespoons plus 1 teaspoon granulated sugar, the cocoa powder and 1/8 teaspoon salt. Using a large whisk or hand mixer, whisk in the egg yolks, orange juice and Grand Marnier. Place over a saucepan of simmering water and whisk vigorously until the mixture triples in volume and becomes very thick, 6 to 8 minutes; it will feel very hot to the touch. Remove from the heat and add the chopped chocolate, whisking till smooth. Set the bowl in a larger bowl filled with ice water until chilled.
In a medium bowl, whip the heavy cream with the remaining 2 tablespoons granulated sugar until stiff peaks just begin to form. Fold one-third of the whipped cream into the chocolate mixture. Fold in the remaining whipped cream. Serve each slice of cake with a generous dollop of the chocolate cream.