For cake: Position rack in center of oven and preheat to 325°F. Butter and flour two 9-inch-diameter cake pans with 1 1/2-inch-high sides. Line bottoms of pans with parchment paper rounds. Stir chocolate in top of double boiler set over simmering water until smooth. Remove from over water. Cool to barely lukewarm.
Sift flour and next 4 ingredients into medium bowl. Whisk milk and yogurt to blend in small bowl. Using electric mixer, beat brown sugar, butter, and lavender in large bowl until smooth. Beat in eggs, 1 at a time. Beat in melted chocolate until smooth. Mix in dry ingredients alternately with milk mixture in 3 additions each. Divide batter between prepared pans.
Bake cakes until tester inserted into centers comes out clean, about 30 minutes. Cool cakes in pans on racks 15 minutes. Turn cakes out onto racks; remove parchment and cool completely.
For buttercream frosting: Stir 8 ounces chocolate in top of double boiler set over simmering water until smooth. Cool until barely lukewarm but still pourable. Mix cocoa powder and 7 tablespoons water in heavy small saucepan. Stir over medium-low heat until smooth and thick but still pourable, adding more water by teaspoonfuls if necessary. Cool.
Beat butter, 1/3 cup powdered sugar, and orange peel in large bowl to blend. Add melted chocolate, vanilla, and salt; beat until smooth. Beat in cocoa mixture. Gradually add remaining 3 2/3 cups powdered sugar and beat until frosting is smooth. Mix in Grand Marnier.
Place 1 cake layer on platter. Spread 1 1/2 cups frosting over top of cake. Top with second cake layer. Spread remaining frosting over top and sides of cake, swirling decoratively. Mound chocolate curls in center of cake. (Can be made 1 day ahead. Cover with cake dome and refrigerate. Let cake stand at room temperature 2 hours before serving.)
*Dried lavender blossoms are available at natural foods stores and at some specialty foods stores and farmers' markets.