Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans.
Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
Beat butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each.
Mix in orange zest, orange juice, and vanilla extract.
Alternate adding dry ingredients and buttermilk, mixing until just combined.
Divide batter evenly between prepared pans.
Bake 25-30 minutes until a toothpick comes out clean.
Cool cakes in pans 10 minutes, then transfer to wire racks to cool completely.
Beat butter for frosting until creamy.
Gradually add powdered sugar alternated with heavy cream and orange juice.
Stir in vanilla extract and orange zest.
Beat until light and fluffy.
Place one cake layer on plate and spread frosting on top.
Add second layer and frost entire cake.
Decorate with candied orange slices or white chocolate shavings.
Let cake set 30 minutes before slicing.
Serve chilled or at room temperature, paired with tea or vanilla ice cream.