Prepare the Simple Syrup: Add the water and 2 cups of the granulated sugar to a saucepan. Heat over medium-low heat and simmer for about 3 minutes, until the sugar has completely dissolved and a simple syrup forms.
Cool the Syrup: Remove the syrup from the heat and allow it to cool for 5–10 minutes. Meanwhile, place the remaining 1 cup of granulated sugar in a bowl or on a rimmed plate.
Soak the Cranberries: Add the fresh or frozen whole cranberries to the partially cooled syrup. Let them sit in the syrup for at least 2 hours, or transfer them to an airtight container and refrigerate them overnight.
Coat the Cranberries: Drain the cranberries, then roll them in the reserved granulated sugar until coated. Set them aside and allow them to dry for at least 1 hour.
Prepare the Oven: Preheat the oven to 350°F. Line a standard muffin pan with cupcake liners and set the prepared pan aside.
Combine the Dry Ingredients: Place the flour, baking powder, kosher salt, and 3/4 cup granulated sugar in a large mixing bowl. Whisk until evenly combined.
Mix the Cupcake Batter: Add the softened butter, eggs, vanilla extract, and buttermilk to the dry ingredients. Using a hand mixer, beat until the mixture is smooth and just combined, with no visible streaks of flour remaining.
Add Cranberries and Chocolate: Gently stir the chopped fresh cranberries and white chocolate chips into the batter until they are distributed throughout.
Fill the Cupcake Liners: Divide the batter among the prepared cupcake liners, filling each approximately two-thirds full.
Bake the Cupcakes: Bake at 350°F for 20–22 minutes, or until a toothpick inserted into the center comes out clean or with only a few moist crumbs.
Cool the Cupcakes: Remove the pan from the oven and let the cupcakes cool in the pan for 5–7 minutes. Transfer them to a cooling rack and allow them to cool completely before frosting.
Melt the White Chocolate: Place the 12 oz white chocolate chips in a microwave-safe bowl. Microwave at 50% power in 20–30-second intervals, stirring between each interval, until the chocolate is just melted and smooth.
Cream the Butter: Place the softened butter in a mixing bowl and beat with a hand mixer until creamy.
Blend in the White Chocolate: Pour the melted white chocolate into the creamy butter and mix until the mixture is smooth.
Finish the Buttercream: Add the powdered sugar and beat until the frosting becomes smooth and fluffy.
Pipe the Frosting: Transfer the finished buttercream to a piping bag fitted with your desired tip, such as a closed star tip. Pipe the frosting onto the completely cooled cupcakes. Alternatively, place the frosting in a resealable plastic bag and snip off one corner for piping; this is easier but will not create the same texture as a piping tip. Garnish the Cupcakes: Top each frosted cupcake with a few sugared cranberries. Add a small sprig of fresh rosemary if desired, using it purely as an optional decorative garnish.
