Step 1: Preheat the oven to 350°F. Grease and flour an 8-inch cake pan and set it aside.

Step 2: In a medium-sized bowl, whisk together the flour, baking powder, and salt. Set aside.

Step 3: In a large bowl, beat the butter and sugar together until light and fluffy. Add the eggs, one at a time, and beat well after each addition.

Step 4: Add the flour mixture to the butter mixture in three parts, alternating with the buttermilk. Mix until just combined. Fold in the mashed bananas.

Step 5: Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Step 6: While the cake is baking, prepare the custard. In a medium-sized saucepan, whisk together the sugar, cornstarch, and salt. Add the milk and egg yolks and whisk until well combined.

Step 7: Cook the mixture over medium heat, whisking constantly, until it thickens and comes to a boil. Boil for 1 minute, whisking constantly.

Step 8: Remove the saucepan from the heat and add the butter and vanilla extract. Whisk until the butter is melted and the mixture is smooth.

Step 9: Pour the custard into a bowl and cover it with plastic wrap. Place the bowl in the refrigerator to chill for at least 2 hours.

Step 10: When the cake and custard are both cooled, it's time to assemble the cake. Start by slicing the cake in half horizontally to create two layers.

Step 11: Place one layer of the cake on a cake stand or plate, and spread the chilled custard over the top, leaving a small border around the edge.

Step 12: Place the second layer of cake on top of the custard and gently press down to secure it in place.

Step 13: To make the whipped cream, combine the heavy cream, confectioners' sugar, and vanilla extract in a large bowl. Beat the mixture with an electric mixer until stiff peaks form.

Step 14: Spread the whipped cream over the top and sides of the cake. Use a spatula to create smooth and even layers.