Make the Cupcakes: Preheat the oven to 350°F (176°C) and prepare a standard muffin pan with cupcake liners.
In a medium bowl, whisk together the flour and baking powder; set aside. In a measuring cup, combine the milk and water; set aside.
Using an electric mixer on medium speed, cream the butter and sugar together until light in color and fluffy, about 3 to 4 minutes. Add the vanilla extract and sour cream and mix until well combined. Add 1 of the egg whites and mix until well combined. Add the remaining 2 egg whites and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
Reduce the mixer speed to low; add half of the flour mixture and mix until just incorporated. Add all of the milk mixture and mix until combined. Add the remaining half of the flour mixture and mix until well combined and smooth, scraping down the sides and bottom of the bowl as needed.
Fill the cupcake liners about halfway full. Bake the cupcakes for 15 to 17 minutes, or until a toothpick inserted comes out with a few moist crumbs attached. Remove the cupcakes from the oven and allow them to cool for 5 minutes, then remove them from the pan and transfer them to a cooling rack to finish cooling completely.
Make the White Chocolate Ganache: Put the white chocolate chips in a small microwave-safe bowl. Heat the heavy cream in a small saucepan over medium heat until it just starts to boil, then pour it over the white chocolate chips. Cover the bowl with clear plastic wrap for 3 to 4 minutes, then whisk until smooth. If the white chocolate is not completely melted, microwave the mixture in 10-second intervals, stirring well between each interval, until smooth. Set the white chocolate ganache aside to cool a bit and thicken while you prepare the frosting.
Make the Frosting: Combine the cranberries, water, and the sugar in a small saucepan and cook over medium-high heat for approximately 5 to 7 minutes, until the cranberries begin to pop. Remove the cranberries from the heat and set them aside to cool for about 5 minutes. Transfer the cranberries to a food processor and puree until smooth. Strain the puree though a fine-mesh sieve. You should end up with about ¼ cup (60 ml) of cranberry puree.
Using an electric mixer on medium speed, beat the butter and shortening until combined and smooth, about 2 minutes. Slowly add 1½ cups (173 grams) of the powdered sugar, mixing until smooth after each addition. Add the cranberry puree and mix until well combined. Add the remaining 1½ cups (173 grams) powdered sugar and mix until well combined and smooth. Beat on medium-high speed for 1 minute.
Assemble the Cupcakes: Using a paring knife, cut a hole out of the center of the cupcakes, going about halfway down. Fill with the white chocolate ganache.
Frost the cupcakes with the cranberry frosting using an Ateco #844 or Wilton 1M decorating tip. Leave the top center of the swirl open, rather than fully completing the swirl. Place the sugared cranberries on top of the frosting, filling in the top of the swirl of frosting. The cupcakes should be stored in an airtight container at room temperature for up to 3 days.