In a separate bowl, sift flour, baking powder, baking soda and salt.
Add flour mixture to butter mixture alternating with milk.
Divide dough into two balls. Roll each ball between two sheets of waxed paper until it is about 1/4" thick. As you roll, you will periodically need to pull the paper off of the dough and flip the whole package over and pull the paper off of the other side.
When dough is rolled to desired thickness, lift top sheet of paper and sprinkle liberally with green sugar. Replace the paper and press down lightly to embed the sugar in the dough.
Place the dough sheets (still inside the paper) onto a baking sheet and refrigerate for 1 hour.
Remove dough from fridge. Lift off the paper and cut dough with a cookie cutter (or use a template and trace around it with a knife). Place cookies on an ungreased cookie sheet.
Bake for 10 minutes. Remove from oven and cool on racks.