01 -
Heat your oven to 325 degrees. Grease a 12x17 inch jelly roll pan, line the bottom with parchment paper, grease the paper, and set aside.
02 -
Beat the eggs in a large bowl for about 2 minutes until they turn pale yellow. Add the cake mix, water, and oil, then mix well.
03 -
Divide the batter evenly into 4 bowls. Add a few drops of a different food color to each bowl and stir well.
04 -
Transfer each colored batter into separate piping bags and cut a small corner off each one. Pipe diagonal stripes across the pan, alternating colors. Keep the stripes thin so you don't run out of any color. Use a toothpick to create gentle swirls without mixing the colors too much.
05 -
Tap the pan on the counter to remove air bubbles. Bake for 15 to 20 minutes until the cake is set and springs back when gently pressed. Don't overbake or it will crack when rolling.
06 -
Lay a clean kitchen towel on the counter and dust it generously with powdered sugar. Run a knife around the edges of the baked cake, then flip it onto the towel. Peel off the parchment paper and dust the cake with powdered sugar. Starting at one short end, roll the cake up tightly with the towel inside. Let it cool completely rolled up, about 2 hours.
07 -
Beat the softened butter and vanilla until fluffy. Gradually add the powdered sugar, then add milk one tablespoon at a time until smooth and spreadable.
08 -
Carefully unroll the cooled cake. It might crack slightly. Spread the buttercream evenly over the cake with an offset spatula. Using the towel to help, roll the cake back up tightly without the towel inside. Transfer to a serving plate.
09 -
Use a dry brush to dust off any excess powdered sugar. Cover and chill in the fridge for at least 3 to 4 hours before serving. Store covered in the fridge.