Preheat oven to 350°F. Line bottom of 10-1/2 in. x 15-1/2 in. x 1 in. jelly roll/cookie pan with parchment paper and lightly spray with vegetable pan spray.
In small bowl, combine flour and baking powder. In large bowl, whip egg whites and 1/4 cup of the granulated sugar with electric mixer until stiff but moist peaks form. In large bowl, whip egg yolks, vanilla and remaining 1/4 cup sugar until creamy and lightened in color, about 4-5 minutes. Fold egg whites into the yolk mixture gently but thoroughly. Fold in flour. Pour batter into prepared pan, spreading to edges.
Bake 10 to 12 minutes, or until golden brown at edges. Cool 15 minutes in pan. Loosen sides of cake with a knife; invert cake onto a cooling rack. Peel off parchment paper; flip cake back to original position on cooling rack.
Starting at a long edge, roll up cake with kitchen towel. Transfer rolled cake to cooling rack; cool completely.
In large saucepan, heat whipping cream just to boiling. (Do not boil.) Remove from heat; add chopped candy and let sit 30 seconds. Whisk until melted and smooth. Let ganache set.
In large bowl, beat shortening and butter with electric mixer until light and fluffy. Add 2 tablespoons orange juice and zest; beat well. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, if icing is dry, add additional juice or milk and beat at medium speed until light and fluffy. Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Re-whip before using.
To fill cake, gently unroll cake. Brush evenly with 3 tablespoons of the orange juice. Spread ganache evenly to edges of cake and let set; cover with 1-1/2 cups of orange buttercream icing. Gently reroll cake without parchment paper; cut off ends of roll. Transfer to cake board or serving plate. Brush outside of roll with remaining orange juice.
Use tip 18 to make rosettes, covering the outside of cake with orange buttercream icing. Grate additional Candy Melts or chocolate over top of roll.