To Make Cake: Preheat oven to 350°F. Grease (with butter) and flour a 13- x 9-inch pan. Beat butter with a heavy-duty stand mixer on medium speed until creamy. Gradually add sugar, and beat until light and fluffy, 3 to 4 minutes. Add eggs, 1 at a time, beating just until blended after each addition. Beat in zest, juice, and vanilla.

Stir together flour, salt, and baking soda in a medium bowl; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low speed just until blended after each addition. Pour batter into prepared pan.

Bake in preheated oven until a wooden pick inserted in center comes out clean, 32 to 35 minutes. Cool in pan on wire rack 10 minutes. Remove cake from pan to wire rack, and cool completely, about 45 minutes. Place cake on a serving platter. 

To Make Orange Buttercream: Beat butter, cream cheese, and salt with an electric mixer on medium speed until creamy. Gradually add powdered sugar alternately with orange juice and vanilla, beating on low speed until blended and smooth after each addition. Beat in milk, 1 teaspoon at a time, if needed, until frosting reaches desired consistency. Remove and reserve 1 1/2 cups Orange Buttercream.

To Decorate Cake: Spread top and sides of cake evenly with remaining Orange Buttercream. Place one-third of reserved frosting (about 1/2 cup) in a small bowl, and tint with desired food color gel. Place remaining reserved frosting (about 1 cup) in a pastry bag, and pipe a decorative border around bottom of cake. Pipe desired message on cake with tinted frosting. Break 3 rolled cookies in half. Tie ribbon around middle of each piece. (We used a 3-inch-long piece of 1/4-inch-wide ribbon for each “diploma.”) Arrange on top of cake. Alternatively, top cake with graduation decorations.