Prepare the Filling: In a medium saucepan, combine sugar and cornstarch over medium heat. Whisk in heavy cream, half & half, milk, and banana oil, along with well-beaten egg yolks. Stir constantly for about 5-7 minutes until the mixture thickens and coats the back of a spoon. Remove from heat and add pure vanilla, stirring until well incorporated.

Melt the Chocolate: In a separate bowl, place the semisweet chocolate chips. Measure out 1.5 cups of the hot vanilla filling and pour it over the chocolate. Stir gently until the chocolate is fully melted and smooth, creating a luscious chocolate custard. Let the remaining filling cool in a bowl covered with plastic wrap to prevent a skin from forming.

Make the Whipped Cream: Using an electric mixer, pour in the heavy whipping cream and add powdered sugar and vanilla. Beat on high speed until stiff peaks form, about 3-5 minutes. The whipped cream should be light and fluffy, holding its shape.

Assemble the Pie: Start with your chocolate cookie crust. Spread ½ cup of the cooled vanilla custard as the first layer in the crust, followed by a layer of banana slices. Press the slices gently. Repeat layering with more custard and banana slices, ending with the chocolate custard layer on top.

Top with Whipped Cream: Generously spread the prepared whipped cream over the top of the chocolate custard layer. Chill the assembled pie in the refrigerator for at least 2 hours to set.

Serve and Enjoy: Once chilled, slice the pie into generous servings. For added flair, garnish with banana slices or a light drizzle of chocolate sauce.