First prepare the white dough by adding the soft butter to a mixing bowl together with the sugar and ½ amount of one scrambled egg and vanilla essence. Mix and whisk the content to a smooth pastry.
Then sieve the flour into the pastry mixture so that it's easier to mix the whole content into a dough. Create a smooth looking dough. It will form itself into a lovely smooth dough at some point as you can see in the video. Make sure to not use your hands for that, use a cake spatula, because the body heat will just make it all too soft.
Then form into a brick of about 2 inch thickness and wrap into a clean wrap. Place into the fridge for at least 30 minutes so that the dough gets hard again.
Then you prepare the black dough the same way. Add the butter to the mixing bowl with the remaining egg and the sugar as mentioned in the ingredients list. Mix and whisk to a smooth paste
Then sieve the flour and cocoa powder into the bowl with the pastry and repeat the process of creating a smooth dough with the baking spatula.
Shape the dark dough into the same size as the white dough (that's important for later!) and wrap into clean wrap as well and place into the fridge for 30 minutes minimum as well.
Take out the 2 bricks of dough from the fridge and unwrap one first and then the other one. Start with the white one, cut into half lengthwise, and then again the half pieces into half so that you have 4 regular sized pieces. Do the same with the black brick.
Then take one white and one black into your hand and press them together. Do that once again with 2 other pieces and then place the two sets as shown in the video together but in a way so that you can see a chess motive emerge. You could continue adding rows that way for a bigger cookie in length if you like by cutting that brick into half.
Make sure to press the whole stitched together block of white and black so that each surface is equal and smooth and so that the parts stick well together in the center. The aim is to have almost straight looking cookie walls. Then wrap the block of dough once again into clean wrap and keep in the fridge for another 30 minutes so that later you can cut it easily into equal sized slices. The dough gets soft in time with the warmth of the hands, that is absolutely normal.
just before taking out the dough from the fridge, preheat the oven to 180 Celsius/356 Fahrenheit so that the oven gets hot enough.
After the block has cooled again in the fridge and is hard enough, take it out of the clean wrap and cut into about 1 cm/0.4 inch thick slices so that you have a few cookies.
Keep a baking tray ready with a baking paper and place the cookies 2 cm/0.8 inch apart from each other onto the baking paper. Bake at 180 Celsius/356 Fahrenheit for about 10-12 minutes. Keep an eye on your cookies in the oven and reduce or add 1-2 minutes depending if the cookies are over baking (turning yellow on the white cookie side) or are under baked. The cookies should be baked but should not change color.
Once bake let cool and directly store afterwards. For a dry climate use a cookie tin box with baking paper and some apple skin to lock the moisture into the cookies. For a humid climate, keep the cookies in an airtight container.