Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, baking powder, baking soda, and salt.

In a separate bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Stir in orange zest.

Pour batter into prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

In a large bowl, cream together butter and powdered sugar until smooth. Add heavy cream, vanilla extract, and orange zest. Beat until light and fluffy.

Once the cake is cool, frost the top of one layer with frosting. Top with the second cake layer and frost the top and sides.

Garnish with additional orange zest, if desired.