2 cups semisweet chocolate chips, plus more for topping
1/2 cup whole milk
1 tablespoon chocolate cream liqueur
1 teaspoon vanilla extract
3 cups confectioners' sugar
1 quart heavy cream
Preheat the oven to 350 degrees F. Line a 9-inch pie plate with the pie crust dough; crimp the edges as desired. Prick holes in the bottom of the pie shell. Bake in the oven 11 minutes, or until golden brown. Let cool.
Combine the chocolate chips, milk and chocolate liqueur in a medium saucepan over medium-low heat. Cook, stirring, until smooth, about 1 minute. Mix in the vanilla extract. Let the chocolate sauce cool.
Combine the confectioners' sugar and heavy cream in a mixing bowl. Whip with a mixer to make whipped cream. Gently fold about 1 cup of the whipped cream into the cooled chocolate sauce. Gently fold the remaining whipped cream in two to three more batches. Pour the mousse mixture into the pie shell. Add chocolate chips to the top of the pie. Cover with plastic wrap and freeze a few hours or overnight.