In a large bowl, beat together the butter, powdered sugar, vanilla extract, and salt until well combined.
Slowly beat in the heavy cream as needed until the buttercream is smooth and has reached the desired consistency for piping.
Divide the frosting among four bowls.
Mix one drop of pink coloring into one bowl of frosting, one drop of blue coloring in the second, one drop of green in the third, and one drop of yellow in the fourth.
Adjust the colors as needed.
Place one large (about 16 inch/41 centimeter) piece of plastic wrap flat on the work surface.
Transfer the pink frosting to the plastic wrap in a line long side to long side in the middle of the plastic wrap.
Carefully place the green frosting in a line next to the pink frosting, taking care not to mix.
Repeat with the blue and yellow frostings.
Gently roll up the plastic wrap to bring together the colors and press together just enough to remove any air and tighten.
Place the wrapped frosting in a piping bag with desired tip.
Pipe onto the cooled, room temperature cupcakes.
Top the cupcakes with unicorn white chocolate pieces or desired sprinkles.