Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment. Turn the mixer on medium speed.

Once the egg whites have become frothy and bubbly, turn the speed up to medium high. Very slowly stream in the granulated sugar while the mixer is running, the slower the better. Once all of the sugar has been added, turn the speed up to high and whisk until you reach firm stiff peaks, then turn the mixer off.

In a separate bowl combine the egg yolks with the milk, oil, and vanilla. Mix until the wet ingredients are well combined.

Then sift the flour and cornstarch into the bowl. Mix again until combined.

Scoop about one cup of the egg whites into the bowl with the egg yolks, gently fold together. Then pour all of that mixture into the bowl with the remaining egg whites and fold together until totally combined.

Pour the batter onto the prepared cookie sheet. Spread the batter out evenly. Gently but firmly tap the bottom of the cookie sheet on the counter, this will help to release some of the air bubbles and ensure the batter is level.

Bake in the preheated oven for about 15 minutes. The cake is done when you can gently press a finger on the surface and the cake springs back.

Remove the cake from the oven and let cool for just a couple of minutes. Place a second sheet of parchment onto the top of the cake. Then slide the cake off of the cookie sheet and onto a clean tea towel on the counter.

While the cake is still warm, remove the top piece of parchment paper and carefully begin to roll the cake up with the towel starting at one of the short sides. If you are concerned about the cake sticking to the towel you can sprinkle a little powdered sugar over the top before rolling. Set aside to cool while you make the buttercream.

To make the buttercream, cream together the butter and powdered sugar until light and fluffy. Add half of the marshmallow cream and beat until combined. Add the remaining marshmallow fluff and beat again until combined and light and fluffy.

Separate the buttercream into 3 bowls, one for each of the colors blue, yellow, and pink. Mix each of the bowls until the desired color has been reached and no more white is showing.

Carefully unravel the cooled cake roll. Slice the brown edges off of each side. Gently apply the 3 colors of buttercream in a striped pattern going parallel to the short ends of the cake. You can make as many or as few stripes of color as you'd like. Save any leftover buttercream to use as glue for the Peeps.

Roll the cake back up tightly, gently remove the bottom piece of parchment as you roll. Wrap the rolled up cake in some parchment or plastic wrap or the tea towel again and place in the refrigerator to set for at least 30 minutes.

When you are ready to serve, slice the ragged ends off of the cake roll, dust with powdered sugar and top with a line of Peeps, alternating in color. You can use a little leftover buttercream to help the peeps stick.

Slice and serve. Store in an airtight container in the refrigerator for up to 3 days.