How to Make It
Prepare the Cookies: Beat butter with an electric mixer on medium-low speed until creamy. Add powdered sugar, egg, egg yolk, and vanilla; beat on medium speed until light and fluffy, about 5 minutes.
Whisk together flour, baking powder, and salt in a bowl. Add flour mixture to butter mixture; beat on low speed until combined, about 2 minutes. (Dough will be very soft and sticky.)
Transfer half of dough onto plastic wrap, and wrap tightly, shaping into a 4-inch disk. Repeat process with remaining half of dough. Chill 2 to 12 hours.
Preheat oven to 350°F. Unwrap 1 dough disk, and roll to 1/4-inch thickness on a lightly floured surface. Cut with a 5- x 3 1/2-inch tree-shaped cutter, rerolling scraps once. Place cookies, 2 inches apart, on parchment paper-lined baking sheets.
Bake in preheated oven until set, 8 to 9 minutes, switching pans top rack to bottom rack halfway through baking. (Cookies should not brown.) Cool on pans 2 minutes. Remove cookies to wire racks, and cool completely, about 30 minutes. Repeat process with remaining dough disk.
Prepare the Icing: Beat cream cheese and butter on medium speed until well combined and smooth, about 2 minutes. Add powdered sugar, vanilla, and salt; beat on medium-low speed until smooth, about 3 minutes. Add sour cream, and beat until incorporated.
Spread icing over cookies, and sprinkle evenly with green sparkling sugar.