In a large mixing bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds. Add the granulated sugar, brown sugar, and baking soda. Beat mixture until combined, scraping sides of bowl occasionally. Beat in the eggs, vanilla, and melted chocolate until combined. Beat in as much of the flour as you can with the mixer. Stir in remaining flour and the malted milk powder. Stir in the malted milk balls.