01 - Preheat oven to 350°F. Grease a 9-inch round or square cake pan and line bottom with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until evenly blended.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in vanilla extract until fully incorporated.
05 - Stir sour cream into the butter mixture until smooth and uniform.
06 - Add flour mixture in three portions, alternating with milk, beginning and ending with flour. Mix only until just combined to avoid overworking the batter.
07 - Gently fold cranberries into batter using a spatula, taking care not to crush the berries.
08 - Pour batter into prepared pan, smoothing the top surface evenly. Sprinkle with turbinado sugar and orange zest if desired.
09 - Bake for 35-40 minutes until a toothpick inserted into the center emerges clean without wet batter.
10 - Let cake cool in pan for 10 minutes, then transfer to wire rack to cool completely before slicing and serving.