Bring milk and scraped vanilla seeds and empty vanilla pods to a simmer over medium low heat in a heavy bottomed medium size sauce pan. After 2 minutes, Using tongs, remove the pods from the warm milk. If using vanilla paste- just add to warm milk-omit the whole beans.
Thoroughly combine the sugar, cornstarch and salt in a medium sized bowl. Add in yolks and whisk well until thick paste is formed.
Add a ladle (about 1/4-1/2 cup) of the hot milk to the sugar/egg mixture to temper*. Whisk the milk in gently to incorporate. Add 3-4 more ladles of hot milk, (whisking in between) until yolks are warmed. Turn the saucepan of remaining milk to down to low heat. Gently pour the now tempered egg mixture into the saucepan of remaining hot milk and whisk constantly until custard/ pudding comes to a slow boil and thickens. Small bubbles will form as it thickens. Keep whisking.
Remove the saucepan from the heat and whisk in butter.
Transfer the pudding mixture to a bowl, cover the surface of pudding with plastic wrap to prevent a skin from forming. With the tip of a knife, poke a few tiny holes in the plastic, to allow steam to escape. Let rest for 30 minutes until just tepid.