Melt the chocolate in a heatproof bowl in the microwave oven for about 30 to 40 seconds. Allow it to cool down.
Sift or sieve all purpose flour, cornstarch, salt and baking soda together in a mixing bowl.
Cream butter and sugar in another mixing bowl till it is creamy and pale in colour.
Add vanilla extract and yogurt. Whisk to mix.
Add the flour mixture. Fold in the flour with the butter sugar mixture, using a spatula or spoon.
Cover the dough with a lid or cling film and allow it to chill for 10-15 minutes in the fridge.
Divide the dough into two equal parts. Can use weighing scales for this.
Keep the one part on the side. That is the white or vanilla one.
To the second part of the dough add the melted chocolate and cocoa powder.
Mix well using a spatula. Allow the dough to rest for 5 minutes.
After that if you still find that the dough is too soft to roll into balls then add 1 -2 tbsp of flour of cocoa powder. Add one first.
Working with the white dough first, make about 90 small balls. I used a ½ tsp measuring spoon as it was easy to scoop out the dough.
Do the same for the chocolate dough, make 90 small balls.
Before you begin preparing the swirl cookies, preheat the oven at 180°C.
Prepare the baking trays by lining them with a silicone mat or with parchment paper.
On your worktop place one white, one brown, one white, one brown, one white and one brown ball in the ring form.
Slowly push the balls with your fingers into the middle, closing the gap.
Take the dough into your palm. Roll gently and it will create the swirls.
Press it down very gently between your palms.
Place the shaped dough on the prepared tray.
Repeat the above with the remaining balls. Watch the video for the method.
Bake the cookies for 12-15 minutes.
At first they will appear soft, but on cooling will become a bit hard. Remember these cookies are soft cookies.
Allow the cookies to rest on the hot tray for 5 minutes. Then remove them and place them on a wire rack to cool completely.
Store them in an airtight container.