Preheat the oven to 350 degrees F. Line 18-20 muffin tin cups with cupcake papers. Set aside.
Cream the butter and sugar together until light and fluffy.
Place the white chocolate in the microwave.
Heat in 20 second intervals until completely melted, stirring between each interval.
Add the white chocolate to the sugar mixture.
Add the flour, baking powder, baking soda, and salt, alternating with the milk and vanilla. Mix just until combined. Be careful to not over mix.
In another bowl, beat the egg whites until the beaters begin to leave tracks.
Add in the remaining 2 tablespoons of sugar.
Beat until the egg whites form stiff peaks.
Carefully fold the egg whites into the cupcake mixture.
Divide the batter evenly between the prepared tins, filling each about ⅔ of the way.
Bake for 18-22 minutes, or until a toothpick inserted in the center of the cupcake comes out clean.
Remove from oven, and let the cupcakes cool in the pan for 5 minutes.
Remove to a wire rack and let the cupcakes cool to room temperature.
To make the buttercream, beat the butter and the powdered sugar together on low for 30 seconds.
Increase speed to medium high and continue to beat until light and fluffy.
Beat in the vanilla and heavy cream, adding more heavy cream as needed to make the correct consistency.
Transfer the frosting into three separate bowls.
Add the blue food coloring to one bowl and the red to a second bowl. Leave the third bowl without food coloring.
Mix in the food coloring until well-combined.
Take a piece of plastic wrap and lay it on your counter. Then take the blue color piping bag and spread a line over the plastic wrap. The line should measure approximately the length of your piping bag. It should fit into the bag and fill it almost completely in length.
Repeat the process with the red and then the uncolored frosting.
Repeat the lines for all three colors. You should have 6 total lines of frosting next to each other. Roll the plastic wrap up to form a cylinder.
Tie up the edges. Cut one of the ends and place the cut end into the bottom of another piping bag with the decorating tip.