To make the vanilla cake
1-Preheat the oven to 325 F: Preheat the oven to 325 F. Grease 12×17 inch jelly roll pan, line the bottom with parchment paper, then grease the paper and set aside.
2-In a large mixing bowl beat the: In a large mixing bowl beat the eggs for about 2 minutes or until pale yellow, then add cake mix, water, oil, and mix well.
3-Divide batter evenly in 4 bowls: Divide batter evenly in 4 bowls. Add a few drop of a food colour in each and stir well.
4-Transfer batter into 6 piping bags, and: Transfer batter into 6 piping bags, and cut off a small corner of each bag. Pipe the stripes in a diagonal pattern, until the pan is filled. To make sure you don’t run out of any colors before you finish covering your entire pan, pipe the colors thin, and then if you have batter remaining, you can go back and fill in as needed. Use a toothpick or a skewer to make the swirls, just don’t swirl it too much and make a mess with the color.
5-Tap your pan on the counter, and: Tap your pan on the counter, and then place it into the oven.
6-Bake for 15-20 minutes, or until the: Bake for 15-20 minutes, or until the cake is set. The cake’s surface should be dry and spring back when gently pressed. Also, do not dry it too much, or it will crack while rolling.
7-Lay out a clean, smooth kitchen towel: Lay out a clean, smooth kitchen towel on the counter and dust it generously with powdered sugar. The cake will be sticky, so make sure to dust the kitchen towel generously with powdered sugar.
8-When the cake is baked, run a: When the cake is baked, run a knife around the edges to loosen from the pan. Flip the cake out onto the dusted kitchen towel and remove the parchment paper. Dust the cake with powdered sugar.
9-Roll up the cake with the towel: Roll up the cake with the towel into a tight log, starting at one short end. Keep the cake rolled up until completely cooled, about 2 hours at room temperature.

To make the vanilla buttercream
10-Beat softened butter and vanilla until fluffy: Beat softened butter and vanilla until fluffy. Gradually add powdered sugar. Add a Tablespoon of milk at the time and mix well.

Assembling the cake roll
11-Carefully unroll the cake: Carefully unroll the cake. It might crack a little. Dollop the buttercream with the spoon, then spread with the offset icing spatula.
12-Using the towel to help, roll the: Using the towel to help, roll the cake back up into a log, tightly as you can and transfer on a serving plate or tray. Using a dry clean brush dust off the excess of powdered sugar. Cover and chill in the fridge for at least 3-4 hours, or until ready to serve.
13-Store covered in the fridge: Store covered in the fridge.