Preheat oven to 350°F/180°C. Grease and flour or line with parchment paper an 8-inch or 9-inch round springform pan (the smaller pan makes a taller cake).
In a large bowl whisk together flour, baking powder, baking soda, salt, and spices. Set aside.
In another medium bowl, whisk eggs and sugar until combined. Add oil, buttermilk, vanilla extract, and orange zest, if using, and whisk until combined. Add the mixture to the flour mixture and fold using a rubber spatula just until combined. Fold in apples.
Scrape batter into prepared pan and smooth the top. Top with sliced almonds, if desired. Bake for 35-45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Place pan on a wire rack and allow to cool completely. To serve, sprinkle with powdered sugar, if desired.