Preheat oven to 350°F (175°C).

In a bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, mixing until combined.

Roll the dough into balls and place on a baking sheet.

Bake for 10 minutes or until lightly golden.

Allow cookies to cool completely.

To make the icing, mix powdered sugar, milk, green food coloring, and vanilla extract until smooth.

Decorate cookies with the green icing as desired.