Can you use a sugar substitute

As long as it's the pure form of the sweetener, it might work. Use Sucralose that has not been modified to make it measure cup for cup (the stuff in the little yellow packets for coffee/tea). The maltodextrin in the modified Sucralose does a funky thing to the texture of cakes (think rubbery frisbee). I learned that the hard way. The big challenge is how much to use. Obviously, you will be using significantly less. This recipe wants you to beat the eggs with 1/3 of the sugar and dissolve the remaining in water to be mixed with the melted chocolate. I think this is doable with Sucralose. I have no experience with monkfruit, so I can't offer any advice for that. Good luck!