How to make Custard Buttercream:

In a saucepan combine 10 egg yolks with 3/4 cup sugar and vigorously whisk until smooth. Right away, whisk in 3/4 cup whole milk.
Set over medium heat and while continuously whisking cook until thickened. The custard should coat the back of the spoon nicely (it starts to thicken at 160°F and will curdle if above 180°F). Do not bring to a boil or the mixture will curdle and become unusable.
Allow to completely cool to room temperature.
In an electric stand mixer whip 1 1/2 cups of unsalted room temperature butter until very pale in color and drastically increased in volume, scraping the bottom of the bowl about 2-3 times throughout, about 5-7 minutes.
In 2 additions, whip in the cooled egg yolk custard, minimally whipping in between.
Reserve 1.5 cups of custard buttercream and set aside, this will be used to decorate the outside of the roll later.
How to make Chocolate Angel Food Cake (Egg White only Chocolate Cake):

Prepare all ingredients by measuring them with a kitchen scale. Line a rimmed baking sheet with foil or parchment paper.
Preheat the oven to 350°F with the baking rack in the center.
Add 240 grams of egg whites to an absolutely grease-free, thoroughly cleaned large bowl and start whipping on high until foamy, about 30 seconds.
While continuing to whip, slowly add 300 grams of granulated sugar, then whip for 7 minutes more. Stop whipping when the meringue looks glossy, thick, and balls around the whisk. Underwhipped meringue will be unstable and make the cake dense.
Add 1 tbsp of vanilla extract. Then, set a fine sieve over the bowl with whipped egg whites, and add 120 grams of all-purpose flour and 40 grams of cocoa powder. Sift over the meringue.
Carefully fold the sifted mixture and meringue together until almost no streaks of dry mixture appear. Do not overfold, as that will make the cake dense.
Transfer the batter to a parchment-lined rimmed baking sheet (18in x 12 in) and spread into an even layer. Bake at 350°F for about 10-15 minutes, or until a toothpick inserted into the center comes out clean. Do not open the oven to check sooner than 10 minutes, or the cake will deflate. Remove from the oven and allow to completely cool.
Assemble the Chocolate Custard Roll:

Turn the cooled cake upside down and peel off the parchment paper. Turn the cake back top side up. Spread the custard buttercream into an even layer all over the cake except for 2 inches on the short side of the cake.
Tightly roll the roll towards the end with no buttercream, leaving it seam side down.
Cover in plastic wrap and refrigerate for about 30 minutes to 1 hour. Remove from the fridge, set over a cooling rack and remove the plastic wrap.
Spread about half of the reserved cream all over the roll, about 3/4th cup.
With a sharp serrated knife trim off about 1/2 inch at both ends of the roll, to reveal an even edge.
Make the Chocolate Ganache:

Combine 1/4 cup hot heavy cream and 1/3 cup chopped chocolate or dark chocolate chips and let sit for 1 minute. Stir until smooth. Allow to cool to room temperature or until the ganache is pourable but thick.
Decorate:

Pour the cooled prepared ganache over the roll.
Add the remaining buttercream to a pastry bag (or a ziplock bag) fitted with a star tip and pipe flowers/stars all over the top of the roll. Make some big flowers and small ones.
If you wish, you can add White Chocolate Crispearls to the centers of each piped flower. Any kind of small round chocolate-covered dry blueberry will do though.
With large spatulas transfer the Chocolate Custard Roll to a serving plate and refrigerate until ready to serve.
For nice clean slices, dip a knife into hot water, wipe, and proceed to cut.