Preheat your oven to 350 degrees.
In a large bowl, cream together the softened butter & brown sugar. Mix in 1 egg. Add in the flour, salt, & vanilla extract; mix until just combined. Stir in a good bit of blue food coloring. Add in enough food coloring to reach your desired color.
Spread your cookie dough on a large baking sheet until the thickness reaches ¼-1/2 of an inch.
Bake for 15 minutes, or until the edges of your dough begin to brown slightly. Remove from the oven, & dump your white chocolate chips on top. Place it back into the oven for 60-90 seconds to melt the chocolate chips slightly Be careful to not leave it in there for too long as you don't want your chocolate to seize up. Using a spatula, spread the melty white chocolate all over your cookie until covered.
Sprinkle on your sprinkles. Let the cookie cool for a few minutes so that the chocolate sets. Using a knife or a pizza cutter, slice the cookie into cookie squares.