Prepare the Filling: Combine 1 cup white chocolate and 1/4 cup room-temperature heavy whipping cream in a small microwave-safe bowl. Microwave for 1 minute, then let the mixture stand for another minute.
Smooth the Ganache: Stir the white chocolate and cream together until completely smooth and combined. Press plastic wrap directly against the surface of the ganache and set it aside at room temperature to cool.
Prepare the Pan: Preheat the oven to 350°F (175°C) and line a 12-cup muffin pan with cupcake liners.
Mix the Wet Ingredients: In a large bowl, whisk together the sour cream, water, vegetable or canola oil, room-temperature egg, and vanilla extract or vanilla bean paste until the mixture is fully incorporated.
Combine the Dry Ingredients: Sift the flour, granulated sugar, baking powder, and fine salt directly into the wet mixture. If you do not have a sifter, whisk the dry ingredients together separately before adding them to the wet ingredients. Finish the Batter: Mix the ingredients just until combined and the batter is smooth. Avoid mixing beyond this point once the flour has been incorporated.
Fill the Liners: Divide the batter among the cupcake liners, filling each approximately three-quarters full.
Bake the Cupcakes: Bake at 350°F (175°C) for 18–21 minutes, rotating the pan halfway through baking for more even cooking. The cupcakes are ready when a toothpick inserted into the center comes out with a few moist crumbs. Cool the Cupcakes: Leave the cupcakes in the pan for 10 minutes, then transfer them to a wire rack and allow them to finish cooling completely.
Make the Buttercream Ganache: Combine 1/2 cup white chocolate and 1/4 cup room-temperature heavy whipping cream in a small bowl. Microwave for 1 minute, let the mixture stand for 1 minute, then stir until smooth. Place the bowl in the refrigerator for 10 minutes to cool. Whip the Butter: Beat the room-temperature unsalted butter with a hand mixer or stand mixer fitted with a whisk attachment at medium speed for 1 minute, until smooth and fluffy.
Add the Ganache: Pour the cooled white chocolate mixture into the whipped butter. Add the vanilla extract or vanilla bean paste and fine salt, then mix at medium speed until incorporated.
Add the Sugar: Gradually mix in the powdered sugar at low speed. Scrape the sides and bottom of the bowl with a rubber spatula as needed to ensure everything combines evenly.
Finish the Frosting: Once all the ingredients are incorporated, beat the frosting at medium speed for a couple of minutes until it reaches the desired consistency. If it is too thick, add 1 tablespoon of heavy cream. If it is too thin, add another 1/4 cup of powdered sugar. Smooth the Buttercream: Stir the finished frosting by hand with a rubber spatula until it is very smooth.
Prepare the Piping Bag: Transfer the buttercream to a large piping bag fitted with an open star piping tip. Seal the top with a rubber band or clip and set the bag aside.
Fill the Cupcakes: Once the cupcakes are completely cool, remove a small portion from the center of each cupcake using a small circle cutter or small knife. Fill each cavity with about 1 tablespoon of room-temperature white chocolate ganache.
Pipe the Frosting: Pipe a large swirl of white chocolate buttercream over each filled cupcake using the open star piping tip.
Chill the Frosting: Place the frosted cupcakes in the freezer for about 5 minutes. This brief chilling period helps keep the buttercream firm while the warm ganache is added.
Warm the Ganache: While the cupcakes chill, microwave the remaining white chocolate ganache for 5–10 seconds, just until it becomes thinner and feels warm to the touch.
Drizzle the Cupcakes: Drizzle the warm ganache over the buttercream on each cupcake.
Add the Finishing Touch: Use a microplane to create a light dusting of white chocolate shavings over the cupcakes.
