In a large mixing bowl, beat the egg yolks and powdered sugar together until pale and slightly thickened. If you prefer to cook the egg yolks, place the bowl over a saucepan of simmering water (without touching the water) and whisk for 4 to 5 minutes until slightly thickened and warm to the touch.
In a separate bowl, mix the mascarpone cheese until smooth. Be careful not to overmix, or it can become grainy.
Gently fold the egg mixture into the mascarpone.
Stir in the Baileys Irish Cream until fully combined.
Make the Soaking Mixture
In a shallow dish, combine the Guinness and espresso coffee. Start with ⅓ cup of Guinness, taste, and add more if desired. The more Guinness, the stronger the flavor will be.
Dip and Layer
Quickly dip each ladyfinger into the coffee-Guinness mixture for about 1 second per side (do not let them get too soggy).
Arrange a layer of soaked ladyfingers in a 7x11 or 9x13 dish.
Assemble the Layers
Spread half of the mascarpone mixture evenly over the ladyfingers.
Repeat with another layer of dipped ladyfingers, then spread the remaining mascarpone mixture on top.
Chill and Serve
Cover and refrigerate for at least 6 hours, preferably overnight, to allow the flavors to meld and the texture to set.
Before serving, sprinkle with shaved chocolate, then dust with cocoa powder.