01 - Preheat the oven to 350°F. Grease a 9-inch round or square cake pan with butter and line the bottom with parchment paper for easy removal.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until well blended. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar together with an electric mixer or whisk until light and fluffy, approximately 2–3 minutes.
04 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully incorporated.
05 - Stir in the sour cream until the mixture is smooth and uniform.
06 - Add the flour mixture in three separate additions, alternating with the milk. Begin and end with the flour addition. Mix gently until just combined, being careful not to overmix.
07 - Gently fold the cranberries into the batter using a spatula, distributing them evenly throughout.
08 - Pour the batter into the prepared pan and smooth the top surface. Sprinkle with turbinado sugar and orange zest if desired.
09 - Bake for 35–40 minutes, or until a wooden toothpick inserted into the center emerges clean.
10 - Allow the cake to cool in the pan for 10 minutes, then carefully transfer to a wire rack to cool completely before slicing and serving.