Put the cookies in a ziplock bag, press the air out, seal it and then beat it with a rolling pin to break them into crumbs. You can use a food processor if you prefer
https://cookpad.com/us/recipes/360763-vickys-oreo-cookies-gf-df-ef-sf-nf
Line a swiss roll or jelly roll tin with parchment paper
Pat the cookie crumbs evenly into the bottom of the tin
Put the buttercream in a large mixing bowl. I would use 350g/12oz butter (gold foil wrapped Stork block margarine is dairy & soy-free) to 675g/1.5lbs powdered sugar if I was making my own for this
Mix in the mint extract and a little gel green to make the buttercream a pale pastel green colour
Melt the broken chocolate in a microwaveable bowl on high in 30 second increments until done or over a bain marie on the stove
https://cookpad.com/us/recipes/343048-vickys-homemade-white-chocolate-gf-df-ef-sf-nf
Stir it into the buttercream and mix well, it'll start to stiffen very quickly
Spoon the mixture over the cookie crumb base and smooth over the top with a wet spoon
Top with some chocolate strands if you like. The Dr Oetker ones are free-from. Chill in the fridge for at least an hour
Cut into squares and serve
