Place cornstarch in a medium-sized bowl. Add milk and whisk until cornstarch is dissolved.
½ Cup Cornstarch, ½ Cup Whole Milk
Add the egg and egg yolks to the cornstarch/milk mixture. Whisk lightly until combined.
1 Large Egg, 4 Large Egg Yolks
Combine remaining milk, mango puree and granulated sugar in a medium-sized saucepan. Heat over medium heat until the mixture reaches a low boil - stir often. Remove from heat.
1 Cup Whole Milk, ½ Cup Granulated Sugar, ½ Cup Mango Puree
Slowly pour 1 cup of the hot mixture into the egg mixture, whisking constantly. This step tempers the eggs—if you don’t whisk the whole time, the eggs might scramble.
Return the remaining milk/mango/sugar mixture to a simmer over medium-low heat. Slowly pour the egg mixture into the saucepan, whisking constantly.
Continue to whisk vigorously until the mixture is quite thick (3 - 5 minutes). The continuous whisking is important in this step. The eggs will curdle if not whisked properly - which results in a lumpy, eggy pastry cream. Note: the mixture is thick enough when the cream forms ribbons and becomes difficult to whisk.
Remove from heat. Add the butter and whisk until all the butter is melted.
4 Tablespoons Unsalted Butter
Pour the mixture into the prebaked graham cracker crust. Place plastic wrap over the filling to prevent a crust from forming. Refrigerate for 2 hours or until set.