Preheat the oven to 165 C / 330 F. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment paper.
In a mixing bowl, beat the softened butter and powdered erythritol with a hand mixer until light and fluffy, about 2 minutes.
Add the egg yolk and rosewater. Beat again until fully combined and smooth.
In a separate bowl, whisk the almond flour, chopped pistachios, baking powder, and salt together.
Fold the dry ingredients into the wet mixture until a soft dough forms. It will feel slightly sticky at this stage.
Chill the dough for 15 minutes if it's sticky enough to stick to your hands.
Scoop the dough into 20 balls, about 25 g each, and space them 5 cm apart on the sheet pan.
Flatten each ball gently with your palm since this dough barely spreads while baking.
Press a few extra chopped pistachios into the top of each cookie.
Bake for 12 to 14 minutes, until the edges turn light golden and the centers look set.
Cool on the pan for 5 minutes, then transfer to a rack to cool completely before storing.
