In a large bowl or stand mixer, cream together butter and sugar. Gradually add in flour, about ¼ cup at a time, until dough pulls together. Divide dough in half. Leave one half of shortbread dough in the bowl and place the other half on a lightly floured surface.
Roll out first portion of dough to ¼ inch in thickness. Dip cookie cutters in flour, and cut out shortbread cookies into desired shapes. Place 1 inch apart on a non-greased baking sheet.
Stir in ¼ cup nonpareil sprinkles to the remaining dough and stir until evenly distributed. Repeat step 3 with the second portion of dough.
Bake for 20-25 minutes or until slightly browned. Remove to wire racks and allow to cool completely before icing.
To Make the Easy Sugar Cookie Icing:
In a large bowl, whisk together powdered sugar, corn syrup, 2 tablespoon milk, and vanilla until well combined and no lumps remain.
Lift up the whisk and see if the icing ribbons down and disappears back into the bowl. Stir in milk ½ tablespoon at a time if icing appears too thick. If icing seems too thin, add in more powdered sugar, 1 tablespoon at a time, until desired consistency is reached.
Evenly separate icing into small bowls and add in 1-2 drops gel food coloring until desired color is reached.