Add the graham crumbs, chopped walnuts and coconut to a large bowl and give them a stir.
Add about 1/2 inch water to a medium saucepan, and place a glass bowl overtop (ensure that the bottom of the glass bowl is not touching the water). Turn the burner to low heat.
Add the butter, cocoa and sugar to the glass bowl. Melt together until smooth.
Add in the egg and whisk vigorously (about 5 minutes) until the mixture is thick and glossy.
Pour the mixture over the dry ingredients and stir to combine.
Press the mixture into the bottom of the prepared pan and place in the fridge for 30 minutes.
Beat the butter until soft.
With the mixer on low speed, beat in 2 cups powdered sugar and the mint extract.
Beat in the cream 1 tablespoon at a time, adding a little extra powdered sugar as needed until it reaches the desired thickness and sweetness.
Mix in the food coloring a few drops at a time.
Spread the frosting on top of the base layer and place in the fridge for 5-15 minutes.
Finely chop the chocolate (if not using chocolate chips) and place in a heatproof bowl.
Microwave on medium power for 45 seconds (not high/regular power or it will burn). Remove from the microwave and stir. Repeat the process until melted.
Stir in the oil.
Spread the chocolate over top of the mint layer.
Let the chocolate harden fully (about 2 hours), then cut into bars. Once the chocolate is about 75% hardened - score the chocolate with a thin knife for easier cutting. Do not cut the bars when cold from the fridge, otherwise the chocolate will crack.
Store bars in an airtight contianer for up to 4 days, or inthe fridge for 1 week. Bars can be frozen in an airtight container for up to 2 months, then thaw in the fridge.