Make the espresso coffee and add to a medium bowl. Add the Kahlua (if using). Set aside to cool.
Insert the Butterfly whisk into the Thermomix bowl, add the egg yolks and sugar and mix for 6 minutes, 37 degrees, Speed 4. Pregnancy-Friendly Variation: if you are preparing this while pregnant, the temperature of the eggs should reach 75 degrees celsius. Insert the Butterfly whisk into the Thermomix bowl, add the egg yolks and caster sugar and mix for 4 minutes, 37 degrees, Speed 4. Scrape down the sides of the bowl and mix for a further 2 minutes, 75 degrees, Speed 4. If the machine has not reached 75 degrees by the end of the cooking time, cook for a further 1 minute, 75 degrees, Speed 4.
Set the machine to mix for 3 minutes, Speed 3. While the blades are turning, slowly adding the mascarpone and vanilla bean paste through the MC hole.
Meanwhile, place the coffee, Savoiardi ladyfinger sponges and a 18 X 29 cm tray on a bench, ready to assemble.
Working quickly, dip each side of the sponge fingers into the coffee and Kahlua mixture. * Only dip for 1 second each side, as too long will cause them to become too soggy.
Allow any excess coffee to drip from the sponges and assemble them in a single layer in the bottom of the dish. If there are any gaps, cut some sponge fingers to fit.
Remove the Butterfly from the Thermomix bowl. Spread half of the mascarpone mixture evenly over the sponge layer.
Repeat this process with the final layer of coffee soaked sponges (and chocolate, if using). For the final layer, either spread or pipe the remaining mascarpone over the top.
Cover and refrigerate for at least 4 hours (or overnight is best).
Top with a sprinkle of cocoa powder or dark chocolate shavings, if you prefer.