Lightly butter a baking pan/tin that's 9 x 9 in (23 x 23cm) and at least 1 in (2.5cm) deep. Line it with parchment, at least on the bottom and two sides with slight overhang so you have something to lift out when done. The butter helps the parchment hold in place.
Crush the cookies/biscuits in a food processor. They should be relatively fine crumbs. Add the cocoa powder and sugar and pulse to mix. Melt the butter (I find microwave works well, around 20-30 seconds at a time then stir, but can also do on the stove) and add. Blend so everything is well combined. Scrape down and pulse again if needed. If you don't have a food processor, you can also crush the cookies in a freezer bag with eg a rolling pin, then mix everything in a bowl.
Put the mixture into the prepared pan and press it down evenly, flattening the top with a spatula or wooden spoon. Refrigerate for at least 30 minutes before adding the frosting/icing layer.
When ready, place the confectioner's sugar into a bowl and add the softened butter, peppermint extract and green coloring if using. Add most of the hot water and mix well, adding a little more water if needed. It should be relatively thick but stirable/spreadable. Scoop the mixture over the chilled base and spread evenly, taking care not to mix crumbs into the icing. Chill for around 30 minutes or more to firm up.
Melt the chocolate and oil together (I usually place in the microwave for 30-40 second intervals, stir, heat again then stop once it is almost melted and melt the last using the heat in the dish). Pour the chocolate over the peppermint layer and spread evenly, right to the edges. Try not to over work the chocolate as you will just make it more uneven as it firms up.
Place the pan in the fridge again to harden for at least 30 minutes, if not nearer an hour, until the top is dry and firm. Use the parchment around the edges to lift the whole block from the pan and then cut into pieces. You may find it helpful to score first and/or use a warmed knife to help cut the chocolate layer without it cracking.