Bring milk and first quantity of sugar to a simmer over medium high heat. Whisking constantly to avoid burning.
Combine the eggs, cornstarch, and second quantity of sugar in a heat proof bowl and whisk to combine
Add 1/4 of your hot milk mixture to the egg mixture VERY slowly while whisking constantly to temper your egg yolk mixture. Whisk until smooth.
Slowly add the rest of the milk, whisking constantly. Return mixture to the saucepan and bring to a simmer.
Once your mixture starts bubbling, reduce the heat to medium and continue whisking for 2-3 minutes to ensure the mixture is cooked and properly thickened.
Pour your mixture into a heatproof container and place your butter cubes on top, allowing them to melt. Do not cover.
Allow your pastry cream to cool until just warm to the touch. Stir in your butter and then add in your vanilla extra.
Cover with plastic wrap so that it's touching the surface of the pastry cream to avoid a skin forming. Let cool completely before using.