- Preheat oven to 350°. Line a large cookie sheet with parchment paper and set aside. 
- In a medium mixing bowl, add the flour, baking powder and salt. Whisk to combine. Set aside. 
- In a large mixing bowl, cream together the butter and 1 cup of sugar.  Add the egg and vanilla extract and mix until incorporated. 
- Add half of the dry ingredients to the wet ingredients and mix until well combined.  Add the remaining flour mixture and mix well. 
- Cover and chill the dough for 15 minutes for easier handling. 
- Pour 1/3 cup of sugar into a shallow bowl.  Shape the chilled cookie dough into 1 inch balls and roll in the granulated sugar. 
- Place the sugar-coated cookie dough balls on the lined cookie sheet, about 2 inches apart. (The dough will not expand much as this recipe was adapted from a thumbprint cookie recipe). 
- Bake for 8-10 minutes or until the edges start to turn golden brown. 
- Allow to cool on cookie sheet for 3 minutes before transferring to a wire rack to completely cool. 
- While the cookies are cooling, line a cookie sheet with parchment paper and set aside. Combine the powdered sugar and milk in a small mixing bowl.  Stir until a think glaze forms. 
- Dip the tops of each cookie into the glaze and place back on the lined cookie sheet. 
- Sprinkle with the rainbow nonpareil's and place one jellybean on top of each cookie.