Make the Orange Curd (Make First):
Whisk egg yolks and sugar in a medium saucepan until combined. Add orange zest, orange juice, lemon juice, and salt.
Cook over medium-low heat, whisking constantly, until mixture thickens and coats the back of a spoon, 8-10 minutes. It should reach 170-175°F (77-80°C). Do not boil.
Remove from heat. Add cold butter one piece at a time, whisking until each is incorporated and curd is smooth and glossy.
Strain through a fine-mesh sieve into a bowl, pressing to extract all curd. Press plastic wrap directly onto surface.
Refrigerate until completely cold and set, at least 3 hours or overnight.

Make the Pistachio-Orange Cake:
Preheat oven to 350°F (175°C). Grease three 8-inch round cake pans and line bottoms with parchment.
Whisk together flour, ground pistachios, baking powder, and salt in a medium bowl.
In a stand mixer with paddle attachment, cream butter and sugar until very light and fluffy, 4-5 minutes.
Add eggs one at a time, beating well after each. Add orange zest, orange juice, vanilla, and almond extract and mix until combined.
Add flour mixture in three additions, alternating with milk (beginning and ending with flour), mixing on low just until combined after each addition.
If desired, add a small amount of green gel food coloring to enhance the pistachio's natural pale green—pistachios alone don't create vibrant green cake.
Divide batter evenly among prepared pans (about 14 oz/400g each) and smooth tops.
Bake 25-30 minutes until a toothpick inserted in center comes out clean and cakes spring back when gently pressed.
Cool in pans 10 minutes, then turn out onto wire racks to cool completely.

Make the Swiss Meringue Buttercream:
Combine egg whites, sugar, and salt in the bowl of a stand mixer. Set over a pan of simmering water (bowl should not touch water).
Whisk constantly until sugar dissolves completely and mixture reaches 160°F (71°C)—it should feel smooth when rubbed between fingers, no graininess.
Transfer bowl to stand mixer fitted with whisk attachment. Beat on high speed until stiff, glossy peaks form and bowl feels cool to touch, 10-15 minutes.
Switch to paddle attachment. With mixer on medium-low, add softened butter one tablespoon at a time, beating well after each addition.
The buttercream may look curdled or soupy at some point—this is normal. Keep beating. It will come together into silky, smooth buttercream.
Add vanilla extract, orange zest, and 2 tablespoons orange curd. Beat until smooth and fluffy.
Reserve about 1 cup buttercream in a separate bowl for piping decoration.

Make the Chocolate Ganache:
Place chopped chocolate in a large heat-safe bowl.
Heat cream in a small saucepan until just simmering.
Pour hot cream over chocolate. Let stand 2 minutes, then whisk from center outward until completely smooth.
Whisk in butter and corn syrup until glossy.
Let ganache cool at room temperature, stirring occasionally, until it reaches pourable but thick consistency—it should coat a spoon and hold briefly, about 30-45 minutes. Too warm and it runs off the cake; too cool and it won't spread smoothly.

Assemble the Cake:
Level cake layers if domed, using a serrated knife.
Place first layer on cake board or serving plate. Spread a thin layer of Swiss meringue buttercream, leaving a ½-inch border.
Spoon about ⅓ cup orange curd into center. Spread gently to edges of buttercream border—the buttercream acts as a dam to contain the curd.
Place second layer on top and repeat buttercream and curd.
Top with final layer, bottom-side up for a flat top.
Apply a thin crumb coat of buttercream over entire cake—just enough to seal in crumbs. Refrigerate 30 minutes until firm.
Apply a smooth, even coat of remaining buttercream (reserve piping portion). Use a bench scraper for smooth sides. Refrigerate 30 minutes.

Apply the Ganache:
Place cake on a wire rack set over a baking sheet.
Pour ganache over center of cake, allowing it to cascade down the sides naturally. Use an offset spatula to guide ganache over any bare spots, working quickly before it sets.
For a perfectly smooth finish, let ganache set briefly (1-2 minutes), then use warm offset spatula to smooth sides.
Let ganache set completely at room temperature, about 1 hour, before decorating.

Decorate:
Transfer reserved buttercream to a piping bag fitted with a decorative tip (star, French, or petal tips work well).
Pipe decorative borders, rosettes, or shells around top edge and/or base of cake.
Press chopped pistachios around base of cake or in decorative pattern.
Dust top lightly with ground pistachios.
Garnish with orange zest curls, candied orange slices, and edible gold leaf if using.