Step 1: Prepare the Pistachio Cream

In a large mixing bowl, pour in the double cream. Using an electric mixer (handheld or stand mixer), whip the cream on medium speed until it begins to thicken. Gradually add the icing sugar, continuing to whisk until soft peaks form. Be careful not to overwhip, as this can cause the cream to become grainy.

Once the cream has reached soft peaks, gently fold in the mascarpone cheese using a spatula. It's important to use room temperature mascarpone to avoid lumps in the cream. Mix until just combined, ensuring the mascarpone is evenly distributed.

Finally, add the pistachio paste to the cream mixture. Gently fold it in until the cream turns a pale green color and the pistachio paste is fully incorporated. Avoid overmixing, as this can deflate the cream. Set the pistachio cream aside while you prepare the coffee and sponge fingers.

Step 2: Prepare the Coffee and Sponge Fingers

Brew a strong cup of coffee and allow it to cool completely. You can use espresso or strong brewed coffee, depending on your preference. Pour the cooled coffee into a shallow dish or pan.

One by one, dip the savoiardi sponge fingers into the coffee for just a few seconds on each side. The goal is to lightly soak the biscuits without making them soggy. If you soak them for too long, they will become mushy and difficult to work with.

Step 3: Assemble the First Layer

Select a medium-sized dish (approximately 9x13 inches is ideal, but you can use any shape you prefer). Arrange the coffee-soaked sponge fingers in an even layer across the base of the dish. You may need to break some of the biscuits to fit them snugly into the dish.

Step 4: Create the Layers

Carefully spread a layer of the pistachio cream over the sponge fingers. Use a spatula or spoon to ensure an even coating.

Next, add another layer of coffee-soaked sponge fingers on top of the pistachio cream.

Repeat the layering process, alternating between the coffee-soaked sponge fingers and the pistachio cream, until you have used all the sponge fingers and cream. Ensure that the final layer is a layer of pistachio cream. The number of layers will depend on the size and shape of your dish.

Step 5: Decorate and Chill

To decorate the tiramisu, spoon the remaining pistachio paste into a disposable piping bag or a sandwich bag. If using a sandwich bag, snip a small corner off the end.

Pipe lines of pistachio paste across the top of the tiramisu. Using a toothpick or a skewer, drag through the lines in the opposite direction to create a pretty feathered pattern.

Sprinkle the top of the tiramisu with chopped pistachios for added flavor and visual appeal.

Cover the tiramisu tightly with plastic wrap and chill in the refrigerator for at least 6 hours, or ideally overnight. This allows the flavors to meld together and the tiramisu to set properly.

Step 6: Serve and Enjoy

Remove the tiramisu from the refrigerator about 20 minutes before serving. This will allow it to soften slightly, making it easier to slice and serve.

Sprinkle with additional chopped pistachios before serving, if desired. Slice into squares or rectangles and serve chilled.