For the cookie cake
- Preheat oven to 175* C (347* F) Fan.
- In a large bowl, whisk together the yogurt, eggs, water, vegetable oil and vanilla extract.
- In a separate bowl, combine the cocoa powder, flour, sugar, baking soda and chocolate couverture broken into little pieces.
- Add the dry ingredients to the first bowl with the wet ingredients and mix with a spatula, until all the ingredients are completely combined.
- Grease two round cake pans with butter and dust with flour. They should be large enough to fit 14 sandwich cookies on the bottom of each.
- Divide mixture between cake pans, spreading it evenly with your hands.
- Bake for 30-35 minutes, on the highest rack in the oven.
For the buttercream
- In a mixer’s bowl, add the butter (at room temperature), milk, vanilla extract and icing sugar.
- Beat into a cream but do not overmix because you don’t want it to become too soft.
For the chocolate buttercream
- In a mixer’s bowl, add the icing sugar, cocoa powder, milk, butter and vanilla extract.
- Beat on medium speed for 2-3 minutes, until all of the ingredients are combined and the cream thickens.
- Transfer the first sponge cake onto a serving platter, cookie side down and spread the buttercream over it.
- Cover with the second sponge cake, cookie side down and press down on it lightly to spread the buttercream in between evenly.
- Spread the chocolate buttercream over the second sponge cake.
- Crush the remaining sandwich cookies and sprinkle them over the chocolate buttercream.
- Serve with mint leaves.