Preheat oven to 350 degrees F (175 degrees C). Lightly grease and flour two 8 inch round cake pans. In a small bowl, stir together flour, poppy seeds, baking powder, salt, and mace; set aside.
In a large bowl, cream together 1 cup of butter and 1 cup of sugar until fluffy. Beat in eggs, one at a time, then add grated orange zest and 1 teaspoon vanilla. Stir in the flour mixture, beating at low speed, until well blended. Divide equally into 2 greased and floured 8 inch round cake pans.
Bake in preheated oven for 20 to 30 minutes. Allow cake layers to cool, then split each layer in half horizontally. Put the 4 cake layers together with about 1/3 cup of filling spread between each layer. Spread sides (not top) of cake with remaining filling. Refrigerate. Spread cooled glaze over top of cake and garnish with curls of orange peel. Store cake in refrigerator.
To Make Filling: Combine the confectioner's sugar, 3/4 cup of butter, 2 egg yolks, and 1 tablespoon of vanilla. Beat on medium high speed until light and fluffy (about 2 to 3 minutes).
To Make The Glaze: In a saucepan, combine orange juice, 2 tablespoons sugar, cornstarch, and orange zest. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil. Set aside to cool before spooning over cake.