Preheat oven to 350 degrees F. Line a 9" square pan with foil. Spray bottom and sides lightly with cooking spray.
In a medium bowl, whisk together the flour, baking soda and salt. Set aside.
In a mixing bowl, with an electric mixer, beat room temperature butter for 30 seconds or until creamy. Add the sugar and beat for 2-3 minutes or until lightened and fluffy. Add the egg, then the yolk one at a time, mixing after each addition. Add vanilla and almond extracts and mix until combined.
Gradually add the flour and mix on lowest speed until combined. Stir in rainbow sprinkles and white chocolate chips.
Press cookie dough evenly into prepared pan. Bake for 12-15 minutes or until very light golden brown. Remove from oven and cool completely in the pan.
Place butter in a mixing bowl and beat until creamy, about 1 minute. Add the confectioner's sugar and continue beating until light and fluffy, about 2-3 more minutes. Add milk, a spoonful at a time until you get a creamy consistency.
Divide frosting equally into three bowls. Add food coloring to each bowl to achieve desired shade.
Remove cooled cookie from the pan and remove foil. Place spoonfuls of each frosting color randomly over the top of the cookie. Using an icing spatula, gently connect each pile of frosting to each other, then lightly swirl the top. Be careful not to swirl so much that all of your colors mix together. This technique will give you a soft marbled top with chunks of color beneath it that you'll see when you cut into bars.
Top with assorted sprinkles before the frosting sets. Allow to set, then cut into bars.
*I used unsalted butter in the bars and salted butter in the frosting. If you use unsalted butter in the frosting, add 1/4 teaspoon of salt.
*If you want more control over how your frosting is swirled, see the post for an alternate technique.
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