Arrange a tight layer of malted milk balls (3 cups) over crust. Stir ice cream with cocoa powder and malted milk powder until smooth. Spoon into crust, set on a plate, and freeze 5 hours.
Heat 1/2 cup cream meanwhile until simmering. Put chocolate in a small metal bowl, pour in cream, and let sit until chocolate is melted, about 2 minutes. Stir until smooth. Let cool completely.
Smooth chocolate ganache over top of pie and freeze until set, about 15 minutes.
Whip remaining 1/2 cup cream and swirl onto pie. Chop some malted milk balls and drop onto pie; add a few whole balls. Remove rim and serve immediately.
*Malted milk powder is made from milk, barley malt, and wheat; don't confuse it with Ovaltine, which has other ingredients added. Find it next to the chocolate milk powder in well-stocked grocery stores.
More pie tips:
Let the pie soften for 5 minutes at room temp to make slicing easier.
If you're having trouble freeing your pie from its pan, set it over a bowl of hot water for a couple of minutes and then slide a thin knife between the pan edge and the crust. It should pop right out.
Make ahead: Once the pie is fully frozen through step 3, it keeps for up to 4 days, double-wrapped in plastic wrap. Top it just before serving.