Preheat the oven to 350° F.
For the cake batter: In the bowl of a mixer fitted with the paddle attachment, beat butter at medium speed for about 2 minutes. At a slow speed, gradually add the sugar, then beat for 3 to 5 minutes, or until mixture is homogeneous and creamy.
Add the yolks one by one, beating at low speed after each addition until just incorporated. Then, at low speed, gradually add flour and cocoa powder until you have a homogeneous cake batter, scraping down the batter from the sides of the bowl a few times.
Beat the egg whites until stiff in a separate bowl. Using a spatula, gently fold in the egg whites (do not beat). Grease and flour 8 (10 x 15-inch) jelly roll pans and pour about 1 cup of batter into each. Spread the batter into a very thin and uniform layer covering the bottom of each sheet pan using an offset icing spatula.
Bake the first layer of cake for 3 1/2 to 4 1/2 minutes (but do not let the cake brown) and reserve the others.
MEANWHILE, prepare the cake filling. Heat heavy cream in the microwave until just before boiling point. Pour into a bowl with chocolate chips and stir well until chocolate has melted and is smooth. Reserve close to a warm place so that the chocolate ganache won't harden.
Rolling technique: Remove cake from the oven and place baking sheet on a counter lined with a clean kitchen linen or towel. Gently run a butter knife around the edges to loosen the cake from pan, making it easier to unmold.
Next, spread a kitchen towel on top. Then place an empty and larger baking sheet pan on top of this, right side up, to sandwich the cake and towel between the two pans. Finally, flip the whole assemblage over in order to release the cake from the pan in which it was baked.
Remove what is now the top baking sheet pan, and then gently slide the bottom pan from beneath the cake and towel. Working quickly, spread a thin layer of chocolate ganache on top of the cake and then roll: Starting from one end, pull up the two adjacent corners along the short length of the towel to initiate rolling of the edge of the cake.
Continue supporting/pushing the rolling edge of the cake with the towel beneath as needed until the cake layer is completely rolled. The cake rolls more easily while still warm. Reserve rolled cake layer covered with towel so won't dry.
Bake the next cake and then so on. Each rolled cake is then rolled into the next subsequent sheet of cake that has been spread with the chocolate ganache.
TIP: If any breaks appear in cake, press break with fingers to seal while it is still warm.
Last, slice the 2 edges of the cake to remove rough, imperfect ends. If desired, either use the remaining chocolate ganache as a topping or sprinkle cake with granulated sugar. Garnish with halved strawberries (optional).
Serve (chilled or at room temp) cake roll sliced into thin slices by itself, along with a cup of coffee. or slightly warmed and accompanied by a scoop of vanilla or coconut ice cream.