Preheat oven to 375 degrees F (190 degrees C). Grease a baking sheet.
Mix flour, 1/4 cup sugar, cornstarch, baking powder, and salt together in a bowl. Rub coconut oil into flour mixture using your hands until mixture is crumbly. Add milk, sour cream, and vanilla extract and mix just until dough is combined; fold in pistachios and coconut.
Scoop dough, about 1 teaspoon per cookie, and form into balls; arrange on the prepared baking sheet about 1-inch apart. Mix 1 teaspoon sugar and cinnamon together in a bowl; sprinkle over dough.
Bake in the preheated oven until edges turn light brown, 10 to 12 minutes. Cool cookies on a wire rack.