For the syrup: Add the sugar and water to a small saucepan and place over medium heat. Bring to a boil, stirring until the sugar is fully dissolved. Remove from the heat and let cool to room temperature. The syrup can be made in advance and refrigerated in an airtight container for up to 3 days.
For the pistachio frangipane: Place the powdered sugar and pistachios in a food processor and process until the pistachios are finely chopped. Add the cornstarch and salt and pulse to combine.
Add the butter to a medium bowl and beat with a spatula until smooth. Add the pistachio mixture and mix until incorporated. Add the egg, vanilla, and Amaretto and mix until the frangipane is fluffy.
To assemble the pistachio croissants: Brush the syrup onto both cut-sides of the croissants until the surface is saturated. Spread about 2 tbsp (30 ml) of pistachio frangipane over each croissant half. If using, divide the berries between the bottom halves, then cover them with the top halves. Spoon, then spread about 1 tbsp (15 ml) of pistachio frangipane over the croissant tops, then press some crushed pistachios on top.
Bake the croissants for 15 to 20 minutes, or until the frangipane on top of the croissant is barely turning blond. Transfer the baking sheet to a wire rack and let cool for 10 minutes. Dust with powdered sugar and serve.