To make the custard, set up a double boiler (see recipe notes for additional details). Place the bowl with the milk-cream-sugar-eggs mixture over a saucepan with 1-2" of simmering water and stir constantly, ensuring that you get into any corners or edges of the bowl. This is where an instant read thermometer is a must - you will want to test the temperature every couple of minutes until it reaches 175°F (it should take about 10-15 minutes). If you don't have a thermometer, you can always test your luck and do it by sight. You will want to cook it until it reaches a nappe consistency - dip a spoon in the custard, turn it over and draw your finger across the back. If you have a clean line, it's ready. Remove from heat and add the vanilla extract, stirring to combine. Place a piece of plastic wrap directly on the surface to prevent a skin from forming. Allow to cool on the counter and then transfer to the refrigerator until completely chilled - 8 hours to overnight.