Preheat oven to 350°F. Prepare a standard cupcake pan with 8 paper liners. Set aside. 

In a heat safe bowl, microwave 2 ounces white chocolate in 30 second intervals, stirring between each interval. Set aside, stirring occasionally to cool. 

In a large bowl or stand mixer, cream together butter and sugar until light and fluffy. Add egg whites, followed by vanilla. Scrape the bowl as needed. Mix in cooled melted chocolate, followed by salt and baking powder. Add flour in 1/2 cup portions until all flour has been added. Increase speed after each addition. Finally, add sour cream, mixing until just combined and no streaks remain. 

Divide batter between prepared paper liners using a large cookie scoop. Bake for 20 minutes or until a toothpick inserted comes out clean. Cool in pan until easy to handle, then trasnfer to a wire rack to cool completely. 

Meanwhile, crush candy canes in a plastic bag until only fine crumbs remain. 

To prepare frosting, melt 2 ounces white chocolate in a heat safe bowl. Add butter and cream cheese to a large bowl or stand mixer, beating until light and fluffy and well combined. Mix in cooled melted white chocolate, followed by peppermint extract. Add 1 cup of powdered sugar at a time, mixing well between additions. Continue mixing until fully combined. Transfer frosting to a large piping bag, fit with a piping tip. 

Frost cooled cupcakes. Immediately sprinkle crushed peppermint candy dust over frosting. 

Store cupcakes in an airtight container for up to several days.