In a microwave safe bowl, mix together sugar, eggs, salt, and cornstarch.
Add a little of the milk and stir to combine.
Next, stir in the remaining milk and chocolate chips.
Cook the filling for 6-8 minutes. Stir after 2 minutes, then after each additional minute.
After the filling thickens, stir vigorously to mix. Add the vanilla and butter and continue stirring until combined.
Pour into the prepared graham cracker crust and place on a wire rack to cool.
After the pie has cooled, place in refrigerator until set (1-2 hours).