To make cookie dough, sift flour, baking powder and cornstarch into a medium bowl.
Melt butter; cool slightly in a medium bowl.
Stir in sugar, egg yolks and lemon peel.
Add to flour mixture; mix to a smooth dough.
Cover and refrigerate until firm.
Preheat oven to 400°F (205°C).
Grease baking sheets.
On a floured surface, roll out dough 1/8 inch (3 mm) thick.
Using a 2-1/2-inch (6-cm) fluted cutter, cut out circles from dough.
Cut out center of each circle using a 1/2-inch (1-cm) fluted cutter.
Roll out centers to make more cookies.
Place ring cookies on baking sheets.
Bake 10 minutes or until golden.
Cool slightly on baking sheets then lift off with a spatula.
Cool on a rack.
To make frosting, blend powdered sugar and lemon juice in a small bowl.
Spread frosting over cooled cookies.
Decorate with pistachio nuts before frosting sets.