Mint-Chocolate Cream Pie

This is a delicious recipe for a chocolate cream pie, which I adapted to make a mint-chocolate pie.

¾ C sugar (with semi-sweet chocolate chips, use a heaping 1/2C sugar instead)
¼ C cornstarch
4 egg yolks
½ tsp salt
2 C milk
1 T butter
1 tsp vanilla
7 oz bittersweet chocolate, coarsely chopped (I used 1 C semi-sweet chocolate chips and 1/2 C Guittard green mint chips)
1 pre-baked pie crust

Whisk together in medium saucepan: sugar, cornstarch, yolks, and salt. Whisk in milk. Cook 10-15 min until pudding-like consistency (I did not cook my custard long enough, so it did not set well). Remove custard from heat. Stir-in butter and vanilla. Melt the chocolate in a double boiler (or microwave if using chips). Mix one fourth of the chocolate into the custard, then add the rest. Pour into pie crust, let chill for at least two hours. If desired, top with cool whip and garnish top with chocolate shavings.