1 day before: In a bowl, stir together the sour cream, heavy cream and sugar and let the mixture stand, covered with plastic wrap, at room temperature for 24 hours.
At least 1 hour before: In a metal bowl set over barely simmering water, melt the cocolate, stirring until smooth, and let it cool. Whisk the chocolate, the zest, and the Grand Marnier into the cream mixture and chill the filling, covered, for 1 hour.
Line a buttered jelly-roll pan (15 1/2 x 10 1/2 x 1-inch) with wax paper, butter the paper, and dust the pan with flour, shaking out the excess.
In a small bowl, stir together the buttermilk, poppy seeds, and the zest.
In another bowl, sift together the flour, baking soda, and salt.
In another bowl, cream together the butter and sugars with an electric mixer until the mixture is light and fluffy. Beat in the egg yolks and vanilla until the mixture is just combined.
Add the flour mixture alternately with the buttermilk mixture to the butter mixture, beginning and ending with the flour mixture and beating well after each addition until the batter is just combined.
In another bowl with cleaned beaters, beat the egg whites until they just hold stiff peaks; stir 1/4th of them into the batter to lighten it.
Fold the remaining whites gently but thoroughly into the batter. Pour the batter into the prepared pan, spreading it evenly, and bake the cake in the middle of a preheated 350°F oven for 20-25 minutes, or until the top is golden and the cake springs back when touched lightly. Let the cake cool in the pan on a rack.
Invert the cake onto the rack and remove the wax paper carefully. With a serrated knife, cut the cake crosswise into 3 equal pieces.
In a heavy saucepan, melt the chocolate with the butter and cream over low heat, stirring until the mixture is smooth. Let the glaze cool to room temperature.
On a rack set over a jelly-roll pan, arrange one of the cake layers. Spread it with 1/3 of the filling; top the filling with another cake layer. Spread the cake with half of the remaining filling and top it with the remaining cake layer.
Spread the remaining filling onto the top and sides of the cake and chill the cake for 30 minutes, or until the filling is set.
Pour the glaze evenly over the cake, smoothing it over the sides, and chill the cake for 30 minutes or until the glaze is set.