Preheat oven to 350 degrees and grease 2 9-inch round cake pans. Line bottoms with parchment or wax paper and grease again. Mix flour, cocoa, baking powder, baking soda and salt together in medium-sized bowl, stirring until well-blended and no lumps appear.
In large mixing bowl, beat butter until very creamy. Gradually add sugars, beating until light and fluffy - at least 3 minutes. Beat in eggs, 1 at a time and then add vanilla and beat until blended.
Turn mixer to low and beat in 1/3 of flour mixture, then half of buttermilk, beating only until mixed after each addition. Beat in another 1/3 of flour mixture, then remaining buttermilk and then remaining flour mixture. Beat only until evenly blended - overbeating will toughen cake.
Divide batter evenly between pans, then bang pans on counter several times to remove air bubbles. Bake in centre of 350 degree oven until centre of cake springs back when lightly touched and sides of cake start to pull away from pan. This should take between 30 and 35 minutes. Let cakes cook in pans on rack for 10 minutes before turning out. Remove paper and cool cakes completely on racks. When ready to assemble cake, slice each half horizontally so you will have 4 layers.
Filling: Combine frozen concentrate, sugar, gelatin in a small saucepan. Cook over medium heat, stirring constantly until sugar and gelatin are disolved - about 5 minutes. Remove from heat and stir in orange peel and 1/4 cup of the liqueur. Press sheet of waxed paper into surface of orange mixture and refrigerate just until it no longer feels warm to the touch - about 20 minutes.
In large mixing bowl, whip cream until soft peaks form. Gradually beat in icing sugar until combined. Fold in liqueur mixture until evenly blended.
To Assemble: Place a layer of cake, cut side up on a serving plate, Brush with 1 Tbsp of liqueur. Spoon a scant 1/5 of filling onto middle of layer and gently spread almost to edge. Top with a cake layer, brush with 1 Tbsp liqueur and then spread with scan 1/5 of filling. Repeat until all layers have been added. Use remaining frosting to frost top and sides of cake. Refrigerate immediately for at least 4 hours - preferably overnight, so flavours blend. Serve at room temperature.