Beat 4 egg yolks with half of the sugar (35g) until pale and fluffy and sugar fully dissolves, ideally in a Stand mixer
Mix in other wet ingredients: milk and oil
Sift flour and corn starch in and fold just until combined
Using your Stand mixer whip 4 egg whites with cream of tartar (if using) on low speed for 2-3 minutes then turn on higher speed and continue whipping and adding the rest of the sugar (35g) in small amount at a time. Whip for about 5 more minutes just until shiny stiff peaks, do not overbeat
Fold ⅓ of the meringue into the egg yolk flour mixture. Once incorporated, fold the rest of the meringue in very gently with a rubber spatula. The mixture should stay very airy.
Carefully pour the batter into the baking pan and very gently smooth the top with an offset spatula
Bake for 25 min
Prepare a Cooling rack with a kitchen towel on top dusted with powder sugar
Flip the cake over to the kitchen towel and while it's still hot, roll it up with the kitchen towel in between the roll
Let the cake roll cool to room temperature in this rolled format
Whipped Mascarpone
Make the filling while cake is cooling
Beat mascarpone, heavy cream and powder sugar until smooth and fluffy, about 1-2 minute with Electric hand mixer. Once it has reached the right consistency, do not overbeat as it can become runny.
Place frosting into the fridge until cake is cooling to further set
Assemble
Make sure sponge is on room temp and filling is cold and stable, can hold its shape
Make a small amount of strong espresso
Gently unwrap the cake roll, drizzle coffee all over it, the sponge should get the coffee flavour all over without getting super wet
Spread an even layer of whipped mascarpone on the sponge with an Offset spatula. Leave some whipped mascarpone for later usage to frost the cake roll
Carefully roll up the cake from the short edge using the kitchen towel to help push the cake roll onto itself
Wrap the cakeroll into plastic wrap and place into the fridge for min 4 h to set
After 4 h unwrap the cakeroll, spread some more whipped mascarpone on the top evenly and generously dust with cocoa powder