Heat oven to 350°F. Grease and line a 9-inch springform or square pan with parchment.
Toss cranberries with 1 tbsp flour until lightly coated; set aside.
Whisk 2.1 cups flour with baking powder and salt in a bowl.
Cream butter and granulated sugar until pale and fluffy, about 3 minutes.
Beat in eggs gradually, then mix in vanilla and orange zest.
Mix in dry ingredients in two additions, alternating with sour cream and milk. Stop when just combined.
Fold in floured cranberries gently to avoid streaking.
Spread batter in the pan and level the top. Sprinkle with turbinado sugar.
Bake 42–48 minutes, until golden and a tester comes out clean.
Cool 15 minutes, then release from the pan and cool completely before slicing.