Lightly oil an 8-inch square metal cake pan and line it with plastic wrap.

Scrape the seeds from the vanilla bean into a saucepan.

Add the sugar and 1/2 cup water to the saucepan.

Stir over medium heat until the sugar dissolves completely.

Boil the sugar syrup without stirring for approximately 3 minutes, or until it thickens into a syrupy consistency.

Dissolve the instant coffee granules in the boiling water.

Stir the dissolved coffee into the sugar syrup and mix well.

In a large bowl, beat the egg yolks and Marsala wine together using an electric mixer until well combined.

Gradually beat in the coffee syrup in a steady stream while continuing to mix with the electric mixer.

Continue beating the mixture for approximately 6 minutes, or until it becomes thick and has tripled in volume.

In a separate chilled bowl, whip the heavy cream using an electric mixer until soft peaks form.

Gently fold the whipped cream into the egg yolk mixture along with the yogurt and 3/4 cup of chopped pistachios.

Spread the semifreddo mixture evenly into the prepared cake pan.

Sprinkle the remaining chopped pistachios over the top of the semifreddo.

Cover the cake pan tightly with plastic wrap.

Freeze the semifreddo for at least 45 hours, or until it is firm.

Invert the pan onto a chilled platter to unmold the semifreddo, then remove the plastic wrap.

Turn the semifreddo right side up on the platter.

Dip a knife in hot water, then cut the semifreddo into squares.

Serve the semifreddo squares immediately on chilled dessert plates.

Sprinkle with additional pistachio nuts and rose petals, if desired, for garnish.