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 thoroughly blended.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Incorporate eggs one at a time, mixing well after each addition. Stir in vanilla extract until combined.
05 - Fold sour cream into the butter mixture until smooth and fully incorporated.
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 incorporate cranberries using a spatula, being careful not to crush the fruit.
08 - Transfer batter to prepared pan, smoothing the 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 or with just a few moist crumbs.
10 - Let cake rest in pan for 10 minutes before transferring to a wire rack. Cool completely before slicing and serving.