Remove from heat and stir in the mint flavor (omit if you used fresh mint leaves). Cook the custard, stirring frequently, until mixture thickens a bit and coats the back of a wooden spoon, about 20–30 minutes. Once the cream mixture is very hot, slowly ladle about half into the egg yolks, whisking constantly. Meanwhile, place the egg yolks into a heat-proof bowl. Place the cream, milk, mint leaves (omit if using mint flavor), maple syrup, and salt in the top of a double boiler (or a heat-proof bowl set over a pot of simmering water).