Prepare the mascarpone cream: In a medium bowl, whisk together the 6 large egg yolks (approximately 100g) and 150g (3/4 cup) of granulated sugar. Whisk vigorously until the mixture is pale yellow and slightly thickened. This process incorporates air and helps dissolve the sugar.

Create a double boiler: Fill a saucepan with about 2.5cm (1 inch) of water and bring it to a gentle simmer over medium-low heat. Place the bowl with the egg yolk and sugar mixture over the saucepan, ensuring the bottom of the bowl does not touch the water. This gentle heating cooks the egg yolks without scrambling them.

Cook the sabayon: Whisk the egg yolk and sugar mixture constantly over the simmering water for about 8-10 minutes, or until it has thickened considerably, turned a pale creamy yellow, and reached a temperature of 71°C (160°F) if using a thermometer. This cooked mixture is called a sabayon. Remove the bowl from the heat and let it cool slightly for about 5 minutes.

Incorporate mascarpone: Add 500g (2.2 cups) of room-temperature mascarpone cheese to the slightly cooled sabayon. Gently fold the mascarpone into the sabayon using a spatula until just combined and smooth. Be careful not to overmix, as this can cause the mascarpone to curdle.

Whip the cream: In a separate, clean, chilled bowl, whip 240ml (1 cup) of cold heavy cream using an electric mixer on medium-high speed until stiff peaks form. Be careful not to over-whip, or it will turn into butter.

Combine the cream mixture: Gently fold the whipped cream into the mascarpone and sabayon mixture in two additions until fully incorporated and the cream is light and airy. This creates the signature creamy texture of Tiramisu.

Prepare the coffee mixture: In a shallow dish or bowl, combine 360ml (1.5 cups) of cooled espresso or strong coffee with 30ml (2 tbsp) of dark rum or Marsala wine, if using. Stir gently.

Dip the ladyfingers: Working one at a time, quickly dip each of the 24-30 ladyfingers (approximately 200g) into the coffee mixture. Dip each side for just 1-2 seconds. They should be moist but not saturated. Over-soaking will make them fall apart.

Assemble the first layer: Arrange half of the dipped ladyfingers in a single layer at the bottom of a 20x20cm (8x8 inch) or similar sized dish. Break ladyfingers as needed to fit snugly.

Add the cream layer: Spread half of the mascarpone cream mixture evenly over the layer of ladyfingers, ensuring it reaches all the edges.

Repeat the layers: Dip the remaining ladyfingers in the coffee mixture and arrange them in a second layer over the cream. Then, spread the remaining mascarpone cream evenly over the second layer of ladyfingers.

Chill the Tiramisu: Cover the dish tightly with plastic wrap and refrigerate for at least 6 hours, or preferably overnight. This allows the flavors to meld and the Tiramisu to set properly.

Dust with cocoa: Just before serving, generously dust the top of the Tiramisu with 30g (1/4 cup) of unsweetened cocoa powder using a fine-mesh sieve for an even coating.

Serve: Cut the Tiramisu into 8 portions using a sharp knife and serve immediately.