*Combine cream, milk, 1/3 cup of sugar, ground cinnamon, vanilla bean paste, and salt in a medium saucepan. Place the saucepan on the stove over medium heat.
*Stir frequently until the mixture comes to a simmer.
Remove the saucepan from the heat and allow it to cool for 10 minutes while occasionally stirring.
*While the mixture is cooling, whisk together egg yolks and 3 Tbsp of sugar in a medium heat-resistant bowl. Whisk until the sugar is beginning to dissolve (about 15-30 seconds)
*Add cornstarch to the egg mixture and whisk until combined and slightly thickened.
When the cream mixture has cooled, SLOWLY drizzle about 1/3 cup of the mixture onto the egg mixture while continuously whisking (to prevent the eggs from cooking). SLOWLY mix the remainder of your cream into the egg mixture while still whisking until the egg and cream mixture are completely combined.
Pour the mixture back into the saucepan and return to the stovetop over medium heat. Continue to whisk frequently until the mixture has thickened.
Once the mixture has thickened, remove from heat and pour the mixture through a strainer to avoid a grainy texture in the pastry cream.
*Whisk in your butter one piece at a time until the butter is completely combined.
Place plastic wrap directly on top of the pastry cream to avoid a film from forming. Allow the cream to cool at room temperature for 30 minutes before placing it into the fridge to chill for 2-4 hours. I would suggest making the pastry cream a day in advance as to make the process easier.