In a bowl, combine egg yolks, sugar, corn starch and matcha green tea powder and whisk. The mixture will look dry and lumpy at first but keep mixing and the corn starch will absorb the moisture and the mixture will become more liquid again.
In a pot combine the milk and vanilla and get over medium heat until just barely simmering.
Then, slowly pour the warm milk into the matcha and egg mixture, whisking continuously to combine. This will temper our egg mixture so that the eggs don't scramble.
Once all of the hot milk has been whisked in, pour everything back into the pot and get it back on low heat, stirring constantly.
Cook and stir until the mixture is thickened which is around the time the first bubble appears under the surface.
Once thick remove from the heat immediately and stir in the butter.
Strain the pastry cream through a sieve into a separate bowl.
Press some cling film firmly against the pastry cream to prevent it from forming a skin and then place the bowl of pastry cream in the fridge to cool.