In small bowl, stir together topping ingredients. Set aside.
Preheat oven to 350 degrees F.
In large bowl, stir together flour and sugar. Using a hand mixer, mix in the softened butter, egg yolk, and vanilla. Press mixture into bottom of a 15x10x1-inch baking pan. Bake 25 minutes.
Remove pan from oven and cool about 5 minutes.
Drop topping by teaspoonfuls at even intervals over the warm crust. Return to oven for about 2 minutes to melt the chocolate.
Remove from oven and use a spatula to spread the topping all over the crust. Cool. While still slightly warm, loosen edges with a spatula. Refrigerate until chocolate topping is firm enough to cut into bars.