In a medium saucepan, add the eggs, orange juice, orange zest, sugar, and salt. Stir over medium heat constantly until the mixture boils and become thick and glossy. About 8-10 minutes
Remove from heat and add the butter. Mix until butter is completely melted
Strain mixture, cover with plastic wrap and chill for a minimum of 2 hours
For Orange Cake
Preheat over to 350 degrees F.
Grease 3 - 9" cake pans
In a bowl, mix the flour, salt, and baking powder. Set aside
Cream butter and sugar together on medium speed for 3-4 minutes or until light and fluffy. Clean the sides of the bowl with the spatula
Add one egg at a time beating well after each addition. Add orange zest as well
Mix milk and orange juice in another bowl
Alternate adding the dry and wet ingredients starting and ending with the dry ones. Make sure that everything is well mixed without overmixing
Pour batter in the 3 prepared pans and bake for 15-20 minutes. Mine were ready in 17 minutes
Remove from the oven and let them cool for 15 minutes before unmolding onto baking racks to cool completely
Cakes should be completely cooled before filling and frosting
For Swiss Meringue Buttercream
Fill a pot 1/4 of the way with water and simmer over low heat
Mix the sugar and egg whites in the bowl of a mixer. Place the mixer over the pot of simmering water. Do not let the water touch the bottom of the mixing bowl
Keep whisking until the sugar is completely dissolved or until the mixture registers 160 F using a candy thermometer. It will take approximately 3 minutes
Remove from the heat and attach the bowl to the mixer
Mix on medium speed using the whisk attachment for 8-10 minutes or until the bottom of the bowl feels room temperature. Do not add the butter if the mixture is warm, otherwise the butter will melt
Switch from the whisk to the paddle attachment and add the butter one cube at a time mixing well after each addition. Take your time with this step and add the butter slowly
Finally add the orange juice
Assembly
Level your cakes with a knife
Pipe a buttercream border around you cake and fill it with 1/2 of the chilled curd. Place the second layer of cake on top and repeat the same step
Place the last layer on top of the cake and frost it all over
You can use a few drops of gel food coloring on one portion of the buttercream and decorate as you wish