Preheat your oven to 350°F. For ease of removing the squares when they are ready to cut, line an 8 x 8 inch square baking pan with parchment paper, allowing some of the paper to hang over the sides (2 pieces cut the width of the pan usually work best so the paper lies flat).
In a medium-sized bowl, mix the melted coconut oil, sugar, cocoa powder, graham wafer crumbs and coconut together until well blended. Press the mixture evenly into the pan and bake for 8 minutes. Remove from oven and let cool completely.
In a medium-sized bowl, whisk together the custard powder and sugar. Add the vegan margarine and mix with a fork or beaters until blended. Slowly add the confectioners sugar, alternating with the plant-based milk until you have a smooth custard or frosting consistency. Spoon over the cooled base. Cover and place in the fridge for about an hour or until cool.
Melt the chocolate pieces in a small bowl. You can do this over a double boiler, or you can put them in a bowl in the microwave for 20 seconds at a time, stirring well each time you check on it. Chocolate burns very easily. Add the oil to the melted chocolate and mix until completely blended. Pour over the chilled bars and, with an offset spatula or the back of a large spoon, gently smooth it evenly over the custard filling. Place it back in the fridge to set up, about an hour.
When cutting into squares, fill a tall mug with hot water. Have a towel or napkin ready. Heat the blade of a sharp knife in the hot water, dry off and then score the chocolate where you plan on cutting it. This helps the chocolate not to crack when you cut the squares right through to the base.