Preheat the oven to 325 °F (165 °C). Grease and flour a medium loaf pan (8x4" Baker's Secret on Amazon). Line the bottom with parchment.
In a small bowl, combine the dried cranberries with the rum. Microwave to heat them for a minute or so, then let stand.
Meanwhile, whisk together the flours, baking powder, and salt in a medium bowl.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and the sugar. Scrape down the sides of the bowl with a spatula.
Add the vanilla, and strain off the rum from the cranberries into the mixer bowl.
Add the eggs, one at a time, scraping as needed and beating well until each egg is absorbed.
Take a scoopful of the dry ingredients and add to the bowl with the cranberries to coat them in flour. Then add one third of the dry ingredients to the mixer bowl. Mix on low to combine, then add half the milk. Scrape down the bowl and mix to combine.
Add half of the rest of the flour, continue mixing, then the last of the milk, and then finally the last bit of flour. Toss in the cranberries and mix to combine.
Pour the batter into the prepared pan, and bake the loaf cake on the middle rack until a cake tester inserted into the centre of the cake comes out clean. It takes 60–70 minutes.
Let the loaf cake cool completely.
For the frosting
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and the icing sugar, then add the rum and mix thoroughly until the frosting is creamy and light.
Smear the frosting over the top of the cake. Top with sugared cranberries (to make them, you can follow this method from Love and Olive Oil).