Measure and sift 100 g of plain flour out of and set aside.
Measure 90g of plain flour and 10g of purple sweet potato powder. Sift together and set aside.
In the bowl of an electric mixer fitted with the whisk attachment, cream the butter and sugar until it becomes a white creamish colour.
Add the egg yolk to the electric mixer and blend the egg yolk on low speed for a minute.
Divide the creamed butter mixture into half. Leave half in the electric mixer bowl. Change the attachment to a flat beater attachment and gradually add the 100g of plain flour with the mixer on low speed.
When all the flour is combined and has become crumbly, take out the dough onto a sheet of cling wrap. Wrap the dough with the cling wrap and refrigerate for 30 minutes.
Repeat step 7 for the remaining creamed butter and purple sweet potato powder combined flour.
After 30 minutes rested in the fridge, take the doughs out and place the plain dough on a clean work surface.
Roll the plain dough out to 10cm (3.9inch) wide and 5mm(0.2inch) thick between two 5mm(0.2 inch) thick board or ruler as the step by step photo indicate.
Roll out the purple sweet potato dough out to same width and thickness.
Trim the both dough edge straight but the purple sweet potato dough 1cm (0.4 inch) shorter than the plain dough.
Place the purple sweet potato dough on the plain dough 5mm(0.2 inch) off the both ends.
Roll the dough together and wrap with cling wrap.
Rest the dough in the fridge for at least 2 hours.
Preheat the oven to 160°C (320°F).
Take out the cookie dough from the fridge. Line a baking tray with a baking sheet.
Slice it about 5mm(0.2 inch) thick and place on the baking tray.
Put the baking tray into the oven and turn the temperature down to 150°C (302°C). Bake for 20 minutes.
Remove baking tray from the oven and let the cookie cool on a cooling rack.
*1 large egg weigh about 50g. *2 All together amount. 100 g used for plain colour dough, 90g used for purple colour dough.