Read through the recipe and notes before starting.

• Pinch the stems off the sage leaves and discard. Arrange the sage leaves into a pile and using a sharp knife, slice them cross-wise into slivers. Add them to a medium saucepan.
• Pour 3 ¾ cups of the milk into the saucepan, reserving 1/4 cup on the side. Split the vanilla bean open lengthwise, and using the dull edge of a knife scrape out the tiny seeds into the milk - drop the pod into the milk if you'd like. Set the pan over medium heat and bring the milk to scalding, don't boil. Turn off the heat once the milk is scalding and cover. Allow the vanilla and sage to steep in the milk for 30 minutes.
• In the meantime, in a large bowl, add the egg yolks, cornstarch, salt, and 1/4 of the reserved cold milk. Whisk well to emulsify and a form a smooth slurry. Taking care to whisk well until all the cornstarch is dissolved.

  This step is important as it's the thickening agent. The texture should be pale yellow, aerated, with no bits or bumps of cornstarch at all. Scrape the sides as needed.
• Strain the milk from the saucepan and discard the sage leaves and vanilla pod, see notes for helpful tip. Return the milk to the pot and under low heat add the sugar, taking care to stir the bottom and sides of the pan until the sugar dissolves and no grainy bits are felt at the bottom, 1-2 minutes. Taste and add more sugar only if desired.

  Turn off the heat just for a moment so you don't accidentally boil while you temper the eggs in the next step.
• Take 1 cup of the hot milk and temper the egg mixture by slowly pouring the milk into the eggs while whisking constantly. The idea is to slowly warm the eggs so they can handle the heat without scrambling.
• Return the heat to medium under the milk. Slowly add the tempered eggs mixture to the saucepan while whisking constantly, take care to get the bottom sides of the pan where the mixture can collect (use a cooking spoon or silicone spatula to check on occasion). Continue to whisk over medium heat until bubbles begin to break the surface in a low boil and the mixture begins to thicken. Be patient, bringing this to a low boil is important. The time it takes will vary and could take upwards to 5-7 minutes or much longer, don't raise the heat. ** SEE NOTES. Once the bubbles break, continue to whisk for a minute more then remove the pan from the heat.

  The consistency should be thicker than milk, somewhere between heavy cream and mayonnaise. When this is thickening you will start to see the tracks of the whisk keep their shape a bit. If you coat the back of a spoon at this point and run your finger over the mixture it will separate without coming back together. If this isn't happening, cook it a little longer. Noting that it will firm up when it cools, so don't overdo it.
• Whisk in the butter until it is well incorporated. Transfer the pudding to a large cool bowl. To avoid skin from forming on top, add a piece of parchment paper or plastic wrap, just touching the pudding's surface. Let it sit on the counter for 30 minutes before refrigerating.
• Chill for 3-4 hours in the refrigerator, or until the pudding sets and is chilled completely through. Serve chilled in individual ramekins or dessert glasses.
• Store in an airtight container in the refrigerator for 3-4 days.