Preheat the oven to 350 degrees F.

In a large bowl using a hand-held mixer or stand mixer with paddle attachment, cream the butter and sugars together on medium speed until fluffy and light in color. Mix in egg, egg yolk, and vanilla. Scrape down the sides as needed. Add the malted milk powder and beat until combined.

In a separate bowl, combine the flour, cornstarch, baking soda and salt. On low speed, slowly mix it into the wet ingredients.

Stir in chocolate chips. Dough will be thick and sticky.

Drop cookies onto a parchment lined baking sheet using a large cookie scoop (about 3 Tablespoons), leaving plenty of space between the cookies (they spread out quite a bit). slightly flatten cookies with the palm of your hand or do optional step below.

Bake for 12 minutes, or until the edges are golden brown. The cookies will be very flat and very chewy. Allow to cool slightly before transferring to a cooling rack.

Optional: After the cookies have been baking for about 5 minutes, use a spoon and make circular motions in the top of the cookie to break up the already melted chocolate chips and flatten the cookies.

Store in an air tight container for up to 5 days.