Line a 8x8-inch baking pan with foil. Lightly butter the foil.
In a medium sized saucepan combine the white chocolate chips, butter, salt, condensed milk, marshmallow creme and pudding mix. Heat on medium low heat until melted and smooth.
Stir in the pistachios and reduce heat to the lowest setting.
Meanwhile, melt one cup of dark chocolate chips in the microwave by heating at 50 percent power for 1-2 minutes.
Pour the hot fudge into your prepared pan. Drizzle the melted dark chocolate over the fudge. Using a toothpick or a butter knife, swirl the dark chocolate into the pistachio fudge.
Cover the pan with plastic wrap and refrigerate until set.
Remove from refrigerator, peel off the foil, and cut into squares to serve.
Notes
Store at room temperature in a tightly sealed container.