To make custard, heat milk in a small pot over medium heat until small bubbles form around the edges. In a small bowl, whisk egg yolks, maple syrup, cornstarch and salt until combined. Slowly pour the hot milk over egg mixture, while whisking, to warm eggs. Transfer mixture back to the pot and place over low heat. Cook, stirring constantly, until mixture becomes very thick. Turn off heat and stir in vanilla. Strain custard through a fine-mesh sieve into a heat-safe bowl. Place plastic wrap over custard bowl and press wrap directly on surface of the custard; refrigerate for 2 hours.