For the Design/Pattern Batter:
Mix the ingredients (without the food coloring) together until smooth. You can use an electric whisk if you'd like.
Divide your batter into smaller bowls. The number depends on how many colors your need, and color each bowl accordingly.
I separated the batter into 2 bowls, black and orange for my design.
Prepare your design to fit the pan, flip the image horizontally (or else it will be backwards) and print it out. On a tray or a large cutting board (my preferred method), place your print out underneath a piece of parchment paper cut to fit in the pan.
Using a small piping tip, pipe your design onto the parchment paper. If you have multiple colors overlapping (like in my Haikyu!! design), you need to freeze each color for about 15 minutes before layering on top. Also, be careful to put your colors down in the right order, front goes first, back goes last.
Freeze your design for about 20 minutes before pouring your cake batter on top. This is a good time to make your batter.
For the Vanilla Roll Cake:
Preheat the oven to 350F (with convection)
Separate the egg yolks from the whites.
In a medium bowl, whisk together the egg yolks, oil, milk, and vanilla together until smooth and slightly thicker.
Using a stand mixer, hand mixer, or a whisk, start making your meringue. Whip the egg whites until foamy. Add your sugar in three stages and keep whipping until you have shiny stiff peaks.
Fold your meringue into the yolk mixture ⅓ at a time. Be careful not to lose all the air.
Sift the flour into the mixture and fold until smooth.
Remove your pattern from the freezer and place into your pan. If it doesn't cover the entirety of your pan, be sure to line it with additional parchment paper.
Pour your batter on top of the pattern and smooth it out.
Bake for 14 minutes and golden.
Remove the cake from the pan to cool completely on a wire rack.
For the Chocolate Charcoal Whipped Cream:
Begin whipping your cream until you reach soft peaks. Add in the corn starch, sugar, cocoa powder, and charcoal.
Continue whipping your cream until you have firm peaks but they are not crumbly.You can add more cream as needed to get the right consistency
Instructions to assemble the roll cake:
When the cake is cooled, lay it pattern side down on a work surface. Carefully separate the cake from the parchment paper, little bits will peel off but it's ok! Leave it laying on the parchment.
Add the whipped cream and spread it evenly across the cake.
Being mindful of the direction of your pattern, begin rolling your cake from one end to the other. Use the parchment paper to guide you. Roll up, wrap tightly in parchment and plastic and place in the fridge to set. *** Do not skip this step, if you cut it now, it will fall apart and not stay rolled up!
Slice and enjoy! The cake will stay moist and delicious if it is stored tightly wrapped in the fridge for 3-5 days.