Preheat oven to 350 degrees F. Grease and lightly flour three 8x1-1/2-inch round baking pans; set aside. Stir together flour, baking powder, and baking soda; set aside.
Beat the 3/4 cup butter with an electric mixer on medium to high speed for 30 seconds. Add the 2 cups sugar, 1 teaspoon vanilla, and 1 teaspoon almond extract; beat until fluffy. Alternately add flour mixture and buttermilk or sour milk, beating on low to medium speed just until combined.
Thoroughly wash and dry beaters. In a medium mixing bowl, beat egg whites until stiff peaks form (tips stand straight). Gently fold beaten egg whites into batter. Fold in the pistachio nuts and the orange peel. Pour batter into prepared pans.
Bake in preheated oven 30 to 35 minutes or until a wooden toothpick inserted near the center of each cake comes out clean. Cool in pans on wire racks for 10 minutes. Remove cakes from pans and cool thoroughly on wire racks.
Combine the 1/2 cup sugar and 2 tablespoons all-purpose flour in a medium mixing bowl; add egg yolks. Beat mixture with a wire whisk until combined; set aside. Heat the 1-1/2 cups milk in a heavy saucepan over medium heat just to boiling. Remove from heat. Gradually beat hot milk into egg mixture with the wire whisk; return entire mixture to saucepan. Cook over medium heat until bubbly, whisking constantly. Cook 2 minutes more. Remove from heat. Add white baking bars, 1-1/2 teaspoons vanilla, and 1/2 teaspoon almond extract. Let stand for 1 minute; stir until smooth. Transfer mixture to a bowl. Cover surface with plastic wrap to prevent skin from forming; cool to room temperature. Beat the 1 cup butter in a medium mixing bowl on medium to high speed until fluffy. Add cooled baking bar mixture, one-fourth at a time, beating on low speed after each addition until combined. Spread between layers and over cake.
If desired, pipe decorative lines along the edges and down the sides of the cake, using a decorating bag fitted with a star tip. Serve immediately or cover and store cake in the refrigerator for up to 3 days. Let stand at room temperature for 30 minutes before serving. Just before serving, if desired, top with Caramelized Pistachio Shards. Makes 12 servings.
Caramelized Pistachio Shards
Preheat oven to 350 degrees F. Line a cookie sheet with aluminum foil; coat foil with nonstick cooking spray. Sprinkle 1 tablespoon chopped pistachios onto prepared foil. Bake in preheated oven 6 to 8 minutes or until lightly toasted. Meanwhile, place 1/3 cup sugar in a heavy 10-inch skillet. Heat over medium-high heat until sugar begins to melt, shaking skillet occasionally to heat sugar evenly. Do not stir. When sugar begins to melt, reduce heat to low. Cook for 2 minutes more or until sugar is melted and lightly golden, stirring as needed with a wooden spoon. Remove pan from heat. Stir in 1/2 teaspoon hot water. Immediately pour over nuts on hot baking sheet. Let cool for 1 minute. Using two forks, gently pull caramel mixture as thinly as possible, lifting slightly as you pull. Let cool thoroughly on foil. Break into pieces; store tightly covered.