Preheat oven to 350 °F or 180 °C, line your cookie sheets with parchment paper.
Sieve together flour and cocoa powder.
Beat butter and sugar until light and fluffy.
Mix together yogurt, olive oil and baking powder to make egg substitute.
Add in egg substitute mixture and beat on medium speed until just mixed. Add in vanilla essence and mix.
Slowly and gradually add in flour and cocoa mixture until combined.
Chill the dough to a minimum of 1 hr. This makes the sugar soak up the moisture/liquid, concentrates the flavor and the fat in the chilled cookie dough takes longer to melt and thus prevents more of cookie spread.
If making immediately, shape the dough into small tennis ball size balls (use an ice cream scooper), chill the balls in the baking tray covered with cling film for around 30 mins.
Once chilled, coat in icing sugar.
Bake in preheated oven for 10-15 mins.
Once baked take out of the oven and cool for 2-3 on the tray and then transfer to the cooling rack.
Let the cookies cool completely then transfer to an airtight container. These are best to be had within 5 days of baking.