Zest your orange into the sugar, and collect 1 Tablespoon of orange juice. Set aside.
Cream the butter into the orange zest and sugar.
Add in the egg and mix until smooth. Add in the orange juice and stir to combine.
Fold in the flour, salt, cream of tartar, and chopped pistachios.
Cover and chill for 1 hour.
Preheat the oven to 350ºF. Line your sheet pan with a silicone baking mat or parchment paper.
Scoop out 2 Tablespoon sized balls onto your mat. As you scoop, pack the dough into the scoop tightly. The balls should be rounded on top and flat on the bottom.
Roll just the rounded top of each ball in sugar, and set it back on the pan (flat side down).
Bake for 15 minutes. Allow to cool for 5 to 10 minutes on the pan before moving to a cooling rack to cool completely.
Store in an airtight container for up to a week, or freeze for up to 6 months.