First, prepare the buttercream. In a small nonstick pot, heat the butter and semisweet chocolate chips until completely melted and smooth. Remove from heat, and refrigerate until completely solid.

Meanwhile, make the cupcakes. Preheat oven to 350F, and line a muffin pan with cupcake liners.

In a microwave-safe bowl, heat the white chocolate and butter in 30 second intervals until melted and smooth. Let cool to room temperature.

In a large bowl, whisk together the eggs, white sugar, brown sugar, and vanilla until smooth and creamy. Whisk in the cooled butter and white chocolate.

Add the salt, baking powder, baking soda, and half of the flour to the liquid ingredients, followed by the sour cream and milk, and finishing with the remaining flour. Mix until batter just comes together.

Fill cupcake liners about ⅔ of the way, and bake for about 16-18 minutes. As soon as the cupcakes come out of the oven, push a truffle in the center of each! Let cool completely before frosting.

Once the initial chocolate chip and butter mixture has completely solidified, transfer it to the bowl of a stand mixer, and beat on high using the whisk attachment until frosting is light and fluffy.

When the cupcakes are cooled off, frost as desired {I used a Wilton 4B tip}, and top with sprinkles. Serve immediately, or chill until ready to serve.