Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving an overhang on the long sides so the baked loaf can be lifted out easily.

Whisk the flour, baking powder, baking soda, and salt together in a medium bowl. Set the dry mixture aside while you prepare the wet ingredients.

In a separate large bowl, whisk the melted, slightly cooled butter with the granulated sugar. Beat in the eggs one at a time, then stir in the vanilla, sour cream or Greek yogurt, and milk.

Add the dry ingredients to the wet ingredients and stir just until no dry flour remains. Do not overmix, since excessive stirring can make the loaf dense.

Toss the cranberries and white chocolate with 1 tablespoon of the measured flour, then fold them into the batter. This light coating helps keep the mix-ins distributed throughout the loaf.

Transfer the batter to the prepared pan and smooth the top. Bake for 50 to 65 minutes, until a toothpick inserted in the center comes out with a few moist crumbs rather than wet batter.

If the top browns too quickly, loosely cover the loaf with foil during the final 15 to 20 minutes of baking. Cool in the pan for 10 to 15 minutes, then use the parchment to transfer it to a rack and cool completely.

For the optional topping, beat the softened cream cheese until smooth. Add the powdered sugar and vanilla, then beat in enough milk or cream to create a spreadable glaze; frost only after the loaf is completely cool.

Spread the topping over the loaf, sprinkle with dried cranberries, and drizzle with melted white chocolate. Let the topping settle briefly, then slice the loaf into about 10 pieces.