Place the butter and sugar in a large mixing bowl. Using a hand mixer like this one, beat the butter and sugar at medium-high speed until light and fluffy. Reduce the speed to low and add one egg at a time, beating gently until combined.
Add the pistachios, cardamom, and vanilla extract. Mix with a spoon until combined.
Stir in the flour mixture until just combined.
Dust the lined baking sheet with flour. Spoon the dough on top. Refrigerate for 30 minutes.
Heat your oven to 350 degrees F, adjusting a rack in the middle.
Take the dough out of the fridge. Using floured hands, form a log that's about 12 inches long (it doesn't need to be perfect. The dough will spread during baking).
Bake for 25 minutes, or until the dough is slightly golden, rotating the baking sheet halfway through.
At this point, the dough should still be quite soft. Remove from the oven and let cool for 10 to 20 minutes. Take a serrated knife and cut the log into ½-inch slices. Arrange the biscotti slices flat in one layer on the baking sheet, keep them about ½ inch apart.
Reduce the oven heat to 275 degrees F. Bake for another 30-35 minutes, turning the biscotti cookies over halfway through until crisp. Remove from the heat and transfer to a wire rack to cool.
Once cooled completely, you can store the biscotti in a loosley covered glass jar or container.
Pro-Tip: the addition of a small amount of butter, although not traditional, produces a richer, more cookie-like texture.
Pro-Tip: Don't skip refrigerating the biscotti dough for 30 minutes before baking the first time. This helps a lot as the dough can be sticky and hard to form
Pro-Tips for Storing: Be sure biscotti are completely cooled before storing. Store in loosely covered glass jar or tin container (what the container is made of makes a difference). This is optional, but it does help, line bottom of jar or container with paper towel
Pro-Tip: If you don't like cardamom, you can use a pinch of nutmeg or cinnamon. You can also replace the pistachios with blanched almonds.