01 - Preheat the oven to 350°F. Grease and flour a 9-inch round cake pan.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon until well blended.
03 - In another bowl, beat butter until creamy. Add eggs one at a time, mixing well after each addition.
04 - Stir in buttermilk, vanilla extract, apple cider vinegar, and orange zest until combined.
05 - Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
06 - Gently fold in golden raisins and walnuts if using.
07 - Pour the batter into the prepared pan and smooth the top evenly.
08 - Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.
10 - Serve plain, dusted with powdered sugar, or with whipped cream.