Crush 9 graham crackers in a food processor or seal them in a plastic bag and crush with a rolling pin until fine—you should have about 1 3/4 cups of crumbs.
Melt 1/2 cup (1 stick) unsalted butter in a small saucepan or microwave, then stir into the graham cracker crumbs until evenly moistened.
Press the mixture into the bottom of an 8- or 9-inch springform pan, then refrigerate for 30 minutes.
Spread 1 (14 oz) can dulce de leche in an even layer over the chilled crust.
Peel and slice 4 medium ripe bananas into 1/4-inch rounds, then arrange them over the dulce de leche.
In a well-chilled bowl, whip 2 cups heavy whipping cream with 1/2 cup powdered sugar until soft peaks form.
Pipe or dollop the whipped cream decoratively over the banana layer.
Dust the top with unsweetened cocoa powder.
Refrigerate the pie for 2 to 3 hours before serving.