Combine the flour, baking powder and salt in a large mixing bowl, give it a quick whisk and set aside until needed.
4 ½ Cups All-purpose flour, 1 Tsp Baking Powder, 1 Pinch salt
In a stand mixer or large bowl, combine the softened butter, sugar and vanilla extract. Now whisk until pale and fluffy. While still whisking add the room temperature eggs one at a time, until smooth and combined.
1 ½ Cups Unsalted Butter, 1 ½ Cups Crystal Sugar, 2 Large Egg, 2 Tsp Vanilla Extract
Continue whisking and add the dry ingredients 3-4 tablespoons at a time until you have a smooth but still sticky cookie dough.
Seperate the dough in two halfs, leave one half in the mixing bowl and wrap the other in clingfilm and pace it in the fridge. Sift the cocoa butter into the remaining half and mix until smooth and well combined. Now wrap the dark half too and keep both in the fridge for 20-30 minutes until firm.
4 Tbsp Cocoa Powder
Cut the firm dough in to further two halfs and roll them into 10*5 inch rectangles. Add flower to both your surface and the dough to avoid sticking. Now place a white rectangle ontop of the cocoa rectangle and trip any overlapping edges. You can knead those back into the remaining dough for the next batch. Roll in to a sausage that is 5 inch long and apx 2 inches thick. Carefully wrap again and place it into the fridge to cool for another 10-15 minutes. This is a good time to preheat your oven and line your baking trays with baking paper.Remove the clingfilm from the dough sausage and slice it with a clean knife into 1/4 Inch slices. Place on the baking tray leaving a little distance as they will spread a bit.
Bake at 170C or 340F for 9-11 minutes with convection, depending on your oven this might be faster or slower so check the colour, when the white part gets a hint of brown, then the cookies are done. Pleace on a cooling rack to cool, this will help avoid trapping moisture under the cookie and thus making it more soggy.
Notes
Important Tips:
Refrigerate the dough until it is firm, it will make rolling and slicing it so much easier. Warm dough will stick, the shapes will deform and the layers will blend into each other.
Clean the knife after every 2nd cut or so, this will avoid smudging the white layer and make the lines clearer.
Use a sharp knife, a blunt knife will put pressure on the dough sausage and will deform it when you press down.
Make sure all your ingredients are at room temperature, this will give you a better blend and fluffier cookie.