Preheat the oven to 320°F (160°C).

In a medium mixing bowl, whisk the egg whites by hand until they become foamy and white with visible bubbles. Add the sugar and stir to combine. Add the ground almonds and stir gently. Mix in the ground pistachios and the Amaretto or vanilla. The mixture will be thick and pliable, similar to modeling clay.

Roll the dough into small balls, about the size of a walnut. Dip one side of each ball into the chopped pistachios, then place, pistachio-side up, on a parchment-lined baking sheet.

Bake for about 15 minutes, until the bottoms are just lightly golden. Do not overbake. Let cool completely. Store in an airtight container at room temperature for up to 5 days.