Heat the oven to 350°F (175°C). Grease the pan and add a parchment sling. Halve cranberries carefully, using them frozen if starting with frozen fruit.
Whisk flour, baking powder, and salt. Rub orange zest into sugar in a separate bowl until the sugar smells fragrant.
Whisk eggs, oil, orange juice, sour cream, and vanilla into the citrus sugar. Keep the wet mixture smooth before adding flour.
Fold dry ingredients into the wet base just until almost combined. Add cranberries and fold gently until evenly dispersed with no dry pockets.
Scrape into the pan, smoothing the top and spreading fruit evenly. Avoid leaving a dense pocket of berries at one end.
Bake 50–60 minutes until a tester in the center shows no raw batter. Tent with foil if the top darkens too far before the middle cooks.
Cool in the pan 15 minutes, then lift onto a rack for about 1 hour. The tart berry pockets stay soft while the crumb settles.
Slice with a serrated knife after cooling. Keep unused bread wrapped closely to protect the cut end and prevent the citrus-scented crumb from drying.
