Pour milk into stainless steel pot and place on medium heat
While the milk is warming up, whisk together the eggs yolks, sugar, and cornstarch until well combined
Once the milk is warm, pour some of the warm milk (approx. 1/4 cup) into the egg and whisk together (this is known as tempering the eggs).
Pour the egg-milk mixture back into the pot and continue to whisk on medium heat until it thickens.
Take the pot off the heat once you start to see bubbles pop from the surface of the pastry cream. Immediately add the butter and vanilla extract and whisk until you have a well emulsified cream
Pour the pastry cream into a bowl (while it is still hot) and cover with plastic wrap. Press the plastic wrap down against the surface of the pastry cream to ensure that it does not form a skin
Leave on countertop to come to room temperature, and then store in the fridge for up to 5 days