You can find the recipe here: http://www.joythebaker.com/blog/2009/12/chocolate-peppermint-sandwich-cookies/
But it’s ok, I’ll explain how we did it!!
We did the filling first, as it needs to cool quite a bit. To do this, bring 1/2 cup of thickened cream to the boil in a small saucepan. Do this on medium heat, then remove it from the hotplate and add 225g finely chopped white chocolate and allow to stand for one minute – make sure the chocolate is completely covered, I used the back of a dessert spoon. Now whisk until the choc is completely melted and 1/4 to 1/2 teaspoon of peppermint extract. We could only find peppermint essence and used 1 teaspoon (I think – we forgot to write this alteration down, so do it bit by bit and taste as you go!).
Transfer to a small bowl and allow to cool until it gets to room temperature. We kept it mostly on the bench, and popped it in the fridge for half hour.
To make the cookies: In a bowl, combine 3/4 cup castor sugar, 1 1/2 cups plus 3 Tbspoons of plain flour, 3/4 cup plus 1 Tbspoon unsweetened cocoa, 1/2 teaspoon baking soda and 1 1/2 teasppons of salt (only add salt if your butter is unsalted).
Add 215 g of cubed, unsalted butter, one cube at a time with a mixer on (this was so handy to do with 2 of us!) The mixture should have a sandy texture, but when we got ours out to roll, it wouldn’t come together, so I added 1 -2 tbspoons of milk.
Transfer the dough to a board and shape into a block. Cut the block into 2 pieces. One piece at a time, roll the dough between 2 sheets of baking paper (I couldn’t find my bluetac, but it’s much easier if you bluetac the baking paper to the bench!) until about 1/8 inch thick (I dare you to try to measure it!!)
Place half of the cookies upside down on a board. Whip the filling with a whisk to loosen it. It lightens in colour and fluffs up. Don’t overdo it though, it will separate. Transfer the filling to a pastry bag or a glad bag with a corner cut out. Pipe onto the upside down cookies and gently press another cookie on top.
These are absolutely awesome – and can be stored for a few days in an airtight container.