Preheat oven to 325 F. Line 2 baking sheets with parchment.
In a bowl, whisk together flour, cornstarch & salt.
In a large bowl, beat butter & sugar with an electric mixer until fluffy & aerated, 4-5 minutes. Add milk & limoncello liqueur; beat for 1 minute.
Sift in half the flour mixture, then fold in with a spatula until just combined. Repeat. Spoon half of the shortbread batter into a piping bag fitted with a large, open star tip.
Pipe batter into 1 1/2-inch wide rosettes, 1 inch apart on a prepared sheets.
Bake shortbread ONLY until edges are lightly browned, 15-16 minutes. Let cool for 3 minutes, then transfer to a rack to cool.
Limoncello Icing
While shortbread cookies are baking, combine powdered sugar & limoncello. Drizzle over warm cookies. While icing is still wet, sprinkle the chopped pistachios over the tops. Allow to set before serving.
Recipe Notes
Trace 1 1/2 inch circles onto parchment paper. Flip the paper over & use circles as a guide when piping rosettes.