Preheat oven to 325ºF.

Prepare a jelly roll pan with shortening and flour. This step will ensure that your cake does not stick. Set aside for later

Prepare cake batter according to the box directions.

Separate batter evenly into 4 bowls.

Add a color of food dye to each bowl and mix until the colors are uniform throughout.

Transfer batter into 4 ziplock bags. Cut a small corner off of each bag.

In a diagonal pattern, squeeze strips of each color until the pan is filled. If you have batter remaining, you can go back and fill in as needed.

Gently shake the pan to even out the cake batter.

Using a toothpick, draw swirls in the batter to create a tie dye look.

Bake for 18 minutes or until the cake is set. Remove from oven and immediately prepare to roll the cake.

Place a towel on a flat surface and sprinkle flour on it.

Flip the cake out of the pan and onto the towel.

Sprinkle a small amount of flour on the top side of the cake and cover it with another towel. Starting on one end, roll the cake up with a towel on each side. Try to keep it as tight as possible.

With the cake tightly rolled, set it aside to completely cool.

When cool, gently unroll it.

To prepare frosting, beat butter and vanilla together. Add powdered sugar and enough milk to reach the consistency you desire.

Spread on the frosting, covering the top of the cake.

Begin rolling the cake from one end, using one towel to help guide you.

When the cake is completely rolled, wrap the towel tightly around it to help hold its form. Refrigerate the roll for at least 20 minutes.

Remove from the fridge and cut into slices.