Preheat the oven to 350 degrees.

In a large bowl cream together the butter and sugar. Add in the egg yolk and vanilla and mix until creamy and smooth.

Add in the flour and salt until a dough forms.

Pour the sprinkles into a shallow bowl.

Using a small cookie scoop, scoop the dough into 1 Tablespoon balls. Roll into the sprinkles and place on the baking sheet 2 inches apart.

Use your thumb or a teaspoon and make an indent in the center of each cookie.

Bake for 10 minutes then remove from oven and cool completely on a wire cooling rack.

To make the icing place the powdered sugar, milk, corn syrup, and vanilla in a bowl and whisk until thick and smooth.

Divide into 3 bowls. Tint one red, one white, and one blue. Carefully use a spoon to pour icing into the center of each cookie. Allow it to harden for 1-2 hours before placing in a container.