Preheat your oven to 350°F and line two large baking sheets with parchment paper. Set aside.
For the chocolate chip cookie dough: In a large bowl mix together the flour, baking powder, baking soda, milk powder and salt.
In a mixer add the butter and both sugars. Cream until the butter is lighter in color and fluffy. Next add in the eggs and vanilla extract. Mix well.
Slowly add the dry ingredients to the butter mixture, mixing just until fully incorporated. Stir in the chocolate chips until fully incorporated.
For the chocolate cookie dough: In a large mixing bowl combine the melted butter and cocoa powder. Mix until well combined. In a separate mixing bowl (or the bowl of a mixer) whisk together the flour, sugar, baking powder and salt.
Add the eggs and vanilla to the cocoa mixture, mix until well combined. Add the cocoa mixture to the dry ingredients and mix until fully incorporated. The mixture is a bit dry and can be a little hard to mix. Keep mixing and it will come together. Using a mixer helps to speed up the process.
Scoop, Assemble, & Bake: Using a 0.5-ounce cookie scoop (or roughly a heaping tablespoon), scoop out all of your chocolate chip dough portions first and place them on your prepared baking sheets. Next, scoop out a matching portion of the chocolate crinkle dough and place it directly side-by-side with each chocolate chip portion.
Roll Together: Pick up one pair of side-by-side dough balls and gently roll them together between your palms so the two doughs merge into one beautifully marbled ball. Place it back onto the baking sheet, spacing the cookies about 2 inches apart.
Flatten & Bake: Lightly press down on the top of each rolled cookie ball with your hand to slightly flatten it out so it bakes evenly.
Bake at 350°F for 12 to 13 minutes, or until the edges are perfectly set and golden brown, but the centers still look slightly underdone. Enjoy!