Preheat your oven to 350°F (175°C) and grease a 9-inch square baking pan with butter or cooking spray.

In a large mixing bowl, cream together the butter, orange zest, and 1 cup of sugar until light and fluffy, about 3 to 5 minutes.

Add in the egg and vanilla extract, mixing until well incorporated and smooth, around 1 minute.

In a separate bowl, whisk together the flour, baking powder, and salt until evenly distributed.

Gradually add the dry mixture to the wet ingredients in three parts, alternating with the buttermilk.

Gently fold in the fresh cranberries until evenly distributed without overmixing.

Pour the batter into the prepared baking pan and sprinkle the remaining tablespoon of sugar on top.

Bake for 30 to 35 minutes or until a toothpick comes out clean or with a few crumbs attached.

Cool the cake in the pan for at least 15 minutes before transferring to a wire rack to cool completely.