Brew 6 double shots of espresso.
You need 375g of coffee liquid total. This will be split into two equal portions of 187.5g for each layer. Allow to cool completely before using.
Add all egg yolks to a ziplock bag and cook, using a sous vide circulator, at 60C for 30 minutes then cool completely.
Mix the sugar and egg yolk using a hand mixer, or stand mixer, at a relatively low speed for 4 to 5 minutes. You should see the mixture increase in volume and begin to notably thicken.
Add the mascarpone in thirds, each time incorporating on lower speeds for 30 to 60 seconds. It doesn't need to be flawlessly mixed.
Pour 187.5g of the coffee mixture in a suitable bowl. Take half the Savoiardi. Dunk the Savoiardi only on the non-sugared side. Then turn upside down and place in the tray. Repeat with all biscuits. You should have a little coffee left over. Starting with the last biscuits you dunked, use a pastry brush to add all the remaining coffee to the biscuits in the layer.
Carefully put half of the mascarpone mixture on top of the biscuits.
Add the remaining 187.5g of coffee to the bowl. Repeat the dunking of the remaining biscuits.
Add the remaining mascarpone cream.
Refrigerate for at least 8 hours, ideally 24. Before serving dust with cocoa powder.