Combine the cornstarch with the water in a small bowl until there are no clumps anymore. Set aside.
In a medium-large pot, add the maple syrup and bring to a gentle boil. Then, keep simmering on medium-high heat for 6 minutes whisking occasionally (put a timer!). Watch carefully as it can overflow easily.
During the last minute of simmering the syrup, warm the coconut milk for 1 minute in the microwave. When the syrup is done simmering, slowly pour the coconut milk into the reduced syrup as you are whisking.
Keep warming up the syrup-coconut mixture until it starts boiling again. Then, still on the heat, slowly add the cornstarch mixture in a thin stream as you are vigorously whisking. Keep whisking and warming for about 30-60 seconds or until the mixture is thick, then remove from the heat and pour over the crust.
Set the dish aside so the caramel can start to firm up. You can also transfer in the fridge for a little if that helps speed up the cooling process. You can touch the caramel to verify when it is firmer and ready to add the chocolate layer (I waited for about 15-20 minutes at room temperature).