Wait to make the custard until the pastry crust is out of the oven cooling.
Whisk together arrowroot and gelatin in small bowl. Into medium saucepan, pour milk, egg yolks, maple syrup, and sea salt. Sprinkle arrowroot mixture over its surface, and whisk again to combine.
Cook over medium heat about 10 minutes, stirring CONSTANTLY until pudding begins to thicken. It will start to steam first, which is your sign that the custard is almost done. Once thickened, remove immediately from heat.
Add butter and vanilla and continue to stir until mixed.