Preheat the oven to 350°F. Line the bottoms of two 9-inch cake pans with parchment paper and coat with nonstick baking spray. Set aside.

In a large mixing bowl, whisk together the cake mix, banana pudding mix, vegetable oil, milk, mashed banana, sour cream, eggs, and extra yolks until smooth and fully incorporated. Divide the batter evenly between the prepared pans.

Bake on the center rack for 25 to 30 minutes, until a toothpick inserted into the middle comes out clean or with a few moist crumbs. Transfer the pans to a wire rack and let the cakes cool completely.

Filling: In a medium bowl, whisk the banana pudding mix with the cold milk for about 2 minutes, until it thickens to a soft-set texture. Refrigerate until ready to use.

Frosting: In the bowl of a stand mixer fitted with the whisk attachment, beat the heavy cream, powdered sugar, vanilla, and salt on medium-high speed until stiff peaks form.

Assemble: Carefully remove the cakes from the pans. Place the first cake layer on a serving plate or cake stand. If the top is domed, slice it off to create a flat surface. Spread the pudding evenly over the layer, then top with banana coins. Place the second cake layer on top.

Frost the entire cake with the whipped cream, smoothing the sides and top with an offset spatula. Garnish with banana slices and wafers. Press crushed wafers onto the sides if desired.