Orange zest gives these buttery cookies a zingy flavor.
Preheat oven to 400. Whisk flour, salt, and zest in a bowl. Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy. Mix in egg and vanilla. Reduce speed to low. Gradually mix in flour mixture.
Transfer batter to a pastry bag fitted with a large star tip (such as Ateco #829). Pipe 2 1/2-inch logs on parchment-lined baking sheets. Whisk together yolk and cream; brush cookies with egg wash. Bake until golden brown, 15 to 17 minutes. Let cool completely on sheets on wire racks.
Cookies can be stored in an airtight container up to 1 week.