In a medium bowl combine the flour, baking powder, baking soda and salt. Set aside.

In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with a hand-held mixer, beat the butter and sugars on high speed until pale and creamy, about 2-3 minutes.

Scrape down the bowl and add the eggs and egg yolk one at a time, mixing after each. Add the vanilla.

Add the dry ingredients and mix on low until incorporated.

Transfer half of the cookie dough base to a large bowl and set aside. Add the malted milk powder and all-purpose flour to the dough remaining in the mixer bowl and mix on low until incorporated. Add the chopped chocolate and mix to combine.

Shape the cookie dough into a disk and cover with plastic wrap. Chill in the fridge for 12-72 hours.

Return the remaining cookie dough base to the mixer bowl (no need to clean). Add the cocoa powder and mix on low until incorporated. Add the chopped white chocolate and mix to combine.

Shape the cookie dough into a disk and cover with plastic wrap. Chill along with the malted chip cookie dough.

Preheat oven to 350 degrees. Line cookie sheets with parchment paper.

Use a medium (number 40) cookie scoop to portion the malted chocolate chip and reverse cookie dough into balls. Roll in your hands until smooth.

Press a ball each of malted chocolate chip cookie dough and reverse chip cookie dough together and gently roll them in your hands to make one big ball.

Press the dough ball into a 1/2-inch thick disk so that one half is malted chocolate chip and one half is reverse chip. Place on a cutting board and cut cookie dough in half perpendicular to the line between the two doughs. Flip one half over and press the two halves back together. You should have a disk with 4 quarters of alternating dough.

Repeat with remaining dough. Place dough disks 6 to a cookie sheet (they will spread to about 5 inches apart).

Bake just until edges of the malted chocolate chip dough turn golden, 14-16 minutes. If some of the cookies spread awkwardly, use a spatula to press the hot cookies back into a circle (do this right after they come out of the oven). Sprinkle with coarse salt, if desired.

Let cool on the pan 5 minutes, then transfer to a wire rack to cool completely.