In a medium bowl, whisk together 1/2 cup granulated sugar, 1/4 cup all-purpose flour, 1/4 teaspoon kosher salt until well combined. Add 4 large egg yolks and continue whisking — this will form a thick, crumbly paste. It's totally okay if the mixture seems dry and clumpy; just make sure that all the ingredients are thoroughly combined.
In a medium, heavy-bottomed saucepan, combine 1 1/2 cups whole milk and 1 teaspoon Pure Leaf Matcha Tea. Whisk over medium-low heat until the matcha dissolves into the milk and it turns a pale green color. Matcha is really hard to dissolve, so don't panic if it's taking a longer time than expected for the matcha to incorporate into the milk. Continue whisking until you see wisps of steam, but nothing more — you don't want the mixture to come to a boil.
Pour a little of the hot milk and matcha mixture into the sugar and egg mixture (from the 1st step); make sure you keep whisking the sugar and egg mixture as you do so, or the eggs might accidentally cook! Continue pouring the milk slowly, whisking continuously until the two mixtures are thoroughly combined.
Once the mixtures are combined, pour everything back into the saucepan. Set the pan back over medium heat and whisk constantly. At first, the pastry cream will look very thin and frothy, but will thicken after a few minutes to a pudding-like texture. Continue whisking until large bubbles start popping on the surface, and immediately remove from heat. Stir in 1 teaspoon pure vanilla extract.
Set a fine mesh sieve over the medium bowl and pour the cream into the strainer. Use a heatproof rubber spatula to stir, pushing it through the strainer. This will catch any bits of cooked egg that maybe in your pastry cream.
Cover the pastry cream with a piece of plastic wrap pressed right up against the surface of the cream to prevent a skin from forming and transfer to the refrigerator to chill completely. The matcha cream will keep in the refrigerator for up to 1 week.