Whisk flour, salt and baking powder together in medium bowl; set aside.
360 g flour, 7 g baking powder, 9 g salt
Place 224g (16 tbsp) of the butter, malted milk powder and dry milk powder in a saucepan over medium-high heat until the butter is melted. Continue cooking, whisking constantly until the butter is dark golden brown and has nutty aroma (about 8 minutes, watch closely as the mixture can go from golden to scorched very quickly). Remove pan from heat and stir the remaining 112 g (8 tbsp) of butter into the hot butter until completely melted.
336 g unsalted butter, 25 g malted milk powder, 25 g dry milk powder
Immediately transfer the browned butter mixture to a large mixing bowl, add both sugars and beat until well combined. Allow to cool to room temperature. Beat the sugar and butter until the mixture starts to thicken, lighten in color and become creamy.
150 g granulated sugar, 330 g dark brown sugar
Add the egg, egg yolks and vanilla, beat until mixture is smooth with no sugar lumps remaining, about 30 seconds.
15 ml vanilla, 1 egg, 2 egg yolk
Using a rubber spatula or wooden spoon, stir in flour mixture until just combined, about 1 minute. Stir in the malted milk balls and chocolate chips giving dough final stir to ensure no flour pockets remain.
270 g semisweet chocolate chips, 280 g malted milk balls
Immediately scoop the mixture into balls using a #30 (2 tbsp) cookie scoop. Cover the balls of dough with plastic wrap and rest in the refrigerator at least overnight or until ready to bake.
When ready to bake, pre-heat the oven to 350°F. Remove the dough balls from the refrigerator and arrange on 2 large, parchment lined baking sheets about 2 inches apart.
Bake until golden brown but still soft, about 10-12 minutes. Cool on the pan for a few minutes before removing to a wire rack to finish cooling. Store in an air tight container.
Notes
You can make these cookies smaller if you like, using a #40 cookie scoop instead. If you do, reduce the baking time to about 8-10 minutes. I would also recommend coarsely chopping/crushing the malted milk balls.
Because of the single egg, this isn't the simplest recipe to cut down. But if you do want to try, I would recommend whisking the egg until it is homogenous in color/texture and then splitting it in half. Typically, half of a large egg would be about 25 g.
Be very careful not to overbake these cookies. Not only will the cookie itself become overly crisp and dry, but the centers of the malted milk balls will melt and compress becoming very hard and unpleasant chunks in the cookie.