Combine the whole milk, ½ cup of the heavy cream, sugar, salt and vanilla in a medium saucepan over medium heat. Cook, stirring occasionally, until the mixture begins steaming and bubbling around the edges. Meanwhile, whisk the egg yolks in a separate bowl and set aside. When milk mixture is steaming and bubbling around the edges, remove 1 cup of it and slowly drizzle the hot milk mixture into the beaten yolks while whisking continuously, to temper the eggs. Make sure not to scramble the eggs by adding all the hot liquid too fast. Add remaining milk mixture and stir to combine into a thin custard. Pour the custard mixture back into the saucepan and continue cooking until it starts to coats the back of a wooden spoon (between 170 and 175 on a candy thermometer) - typically only 1-2 minutes.