Line a cookie sheet with parchment paper or a silicone baking mat and set aside.
In the bowl of a stand mixer, add the butter, brown sugar, and granulated sugar. Beat on medium speed until light and fluffy, 3-4 minutes.
Scrape the sides and bottom of the bowl. Add the eggs, one at a time, beating on medium speed after each addition until well incorporated.
Add vanilla and mix until combined. With the mixer off, add in flour, malted milk powder, salt, baking powder and baking soda. Turn mixer on low and stir just until incorporated.
Scoop dough into 2 tablespoon sized balls and place on the prepared baking sheet.
Cookies can be placed next to one another to fit on the sheet. Cover with plastic wrap and place in the refrigerator overnight.
Remove the cookie dough from the fridge and preheat the oven to 375ºF (190ºC). Line a second baking sheet with parchment paper or a silicone baking mat and spread out cookie dough balls on the baking sheets so they are at least 2 inches apart.
Once the oven is preheated, bake the cookies for 12-14 minutes, rotating the trays front to back and top to bottom halfway through the bake time.
Remove the cookies from the oven and allow to cool on the cookie sheets for at least 10 minutes.
Transfer the cookies to a wire rack set atop a baking tray.
Drizzle cookies with melted chocolate and sprinkle with chopped malt balls.