In a small pot over medium heat, melt butter. Allow butter to a come to a boil. Continuously stir until butter turns an amber color and has a nutty smell, about 5 minutes. Remove from heat and transfer butter to a heat safe bowl. Place in refrigerator for at least 1 hour to allow it to re-harden.
Once butter has hardened, place it in a large bowl, or bowl of stand mixer with sugar and brown sugar. Use mixer to beat on high speed until lightened, about 2-3 minutes.
Add egg, milk, and vanilla, and mix until well combined.
In a small bowl, stir together flour, cornstarch, salt, and baking powder. On medium-low speed, gradually beat into butter mixture until well combined.
Stir in 1/4 cup chocolate chips and 1/4 cup chocolate chunks.