In a large bowl, whisk together egg yolks and 1/2 cup sugar.
In a heavy medium saucepan over moderately low heat, stir together milk, remaining 1/4 cup sugar, and mint. Heat until steaming but not boiling, then remove from heat.
Ladle about 1/2 cup hot milk mixture into egg yolk mixture, whisking constantly to temper the eggs. Slowly stir the egg mixture back into the hot milk, whisking constantly. Place over low heat, and cook, stirring constantly, until mixture thickens enough to coat back of spoon, 5-6 minutes total (do not let boil or custard will curdle). Strain through fine-mesh sieve into large bowl, pressing on and then discarding solids. Whisk in heavy cream and food coloring. Cover and chill until cold, at least 6 hours or overnight.
Process custard in ice cream maker, adding chococlate chips during last minute of churning. Transfer enough of the ice cream to fill the chocolate pie crust. Freeze until hard, about 3 hours.
Cut pie into slices and serve with whipped cream and a sprig of mint on top.