In a medium bowl whisk the cocoa powder and oil until smooth. When smooth, add the melted butter.
In a separate bowl whisk the eggs until light and fluffy. Add the sugar, molasses and vanilla. Stir until combined.
Add the cocoa and oil mixture to the egg mixture. Mix until smooth and glossy.
Pour the chocolate mixture into the prebaked graham crust and bake for 35-40 minutes. The top of the pie will appear crisp and the filling will have set.
Remove the pie from the oven and cool.
Decorate the top of the cool pie with the marshmallow slices. Place pie under broiler and brown the marshmallow slices.