Set up a wire rack with a parchment underneath.
In a medium bowl, whisk together powdered sugar, milk and imitation vanilla extract.
Mix in a few drops of desired food coloring.
Dip doughnut halfway into the icing and place on the wire rack.
In a microwave safe bowl, melt chocolate in 30 second increments.
Prepare a piping bag with a small round tip (I used a #2)
Place about 1/3 of the white chocolate into prepared piping bag.
Pipe 8 lines equally spread apart from the center of the doughnut to the outside of the doughnut.
Make a wide "U" shape connecting two of the lines near the center of the doughnut. Continue connecting the rest of the lines with that "U" shape all the way around the doughnut until you get back to the original "U" shape.
Repeat the process about halfway up the line.
Repeat the process about 3/4 of the way up the line.