- Make the cake: Place a rack in center of oven, and preheat to 350 degrees. Lightly grease and flour three 8-inch round cake pans and set aside.
- Place butter and shortening in a large mixing bowl and beat with an electric mixer on low speed until creamy. Add sugar and beat on medium-low until fluffy, about 30 seconds to 1 minute. Scrape down sides of bowl. Add eggs, one at a time, beating until just combined. Set aside.
- In a medium bowl, whisk together flour, baking soda, and salt. Beginning and ending with flour mixture, alternate adding flour mixture and buttermilk to butter mixture. Set aside.
- Grate 2 teaspoons of orange zest and fold into batter. Squeeze 2 tablespoons of orange juice and stir into batter until well-combined. Pour batter into prepared pans, dividing it equally. Smooth tops with a spatula and place in oven.
- Bake until golden brown and the top springs back when lightly pressed with your finger, about 22 to 27 minutes. Remove pans to a wire rack to cool 15 minutes. Run a knife around edges of pan and loosen cake with gentle shakes. Invert cake once and then again onto a rack to cool completely, right side up, about 30 minutes.
- Make the garnish: While cake cools, place raisins in a glass measuring cup and pour sherry over them. Heat cup in a microwave on high power for 20 seconds or until sherry has warmed. (Alternately, warm raisins and sherry in a small saucepan on stovetop over medium heat, about 5 minutes). Toss raisins and sherry with a fork to coat all the raisins. Set aside. With the oven at 350 degrees, gently toast pecans for 5 minutes. Remove and cool.
- Make the frosting: Place butter in a large mixing bowl and beat on low speed with an electric mixer until soft. Add 1 cup of powdered sugar and blend. Add half of orange juice, zest, and salt and beat until combined, 15 seconds. Add 1 cup more powdered sugar and beat until combined. Scrape sides of bowl with a rubber spatula, and add remaining cup powdered sugar. Beat on low until combined, then increase mixer speed to medium and beat until fluffy, 1 minute.
- To assemble: Place one cake layer on a stand and spread with ¾ cup frosting. Stack second layer on top and spread with ¾ cup frosting. Add top layer. Spread frosting in a thin layer around sides of cake to create a nearly bare-sided look. Pile remaining frosting on top of cake. Strain raisins from sherry and pile in middle of top layer. Scatter chopped pecans in a ring around edges of top layer. Garnish with orange zest, if desired.