Cookies:
4 sticks (1 pound) salted butter, softened
1 1/2 cups granulated sugar
2 large egg yolks
1 tablespoon lemon zest
1 tablespoon lime zest
4 cups all-purpose flour
2 tablespoons lemon or lime juice (or 1 tablespoon of each)
1/2 teaspoon coconut extract

Icing:
3 cups powdered sugar, plus more if needed
2 tablespoons cream of coconut
1 tablespoon lemon zest, plus extra for decorating
1 tablespoon lime zest, plus extra for decorating
1 tablespoon milk
1 teaspoon coconut extract
Dash of salt
1 large egg white
Juice of 1 lemon, plus more if needed
Juice of 1 lime, plus more if needed