Step 1: Prepare Pudding - In a stand mixer, combine 1 cup of heavy cream with the 3.4 oz package of instant vanilla pudding mix. Whip on medium speed until well blended and thickened, about 2 to 3 minutes. Once combined, cover the bowl and chill the mixture in the refrigerator for 30 to 45 minutes.

Step 2: Preheat Oven - While the pudding chills, preheat your oven to 350°F (175°C). Line two cupcake tins with paper liners, ensuring even spacing for optimal baking.

Step 3: Mix Dry Ingredients - In a mixing bowl, whisk together 1.5 cups of cake flour, 1 cup of granulated sugar, 0.25 tsp baking soda, 1.25 tsp baking powder, and 0.25 tsp fine salt.

Step 4: Combine Wet Ingredients - In your stand mixer, mix 0.75 cup of melted unsalted butter with 3 tsp of pure vanilla extract, 1 tsp of banana extract, and 2 room-temperature eggs. Incorporate 0.5 cup of sour cream and 0.5 cup of buttermilk until smooth.

Step 5: Combine Mixtures - Gradually add the dry ingredient mixture to the wet ingredients in the stand mixer, mixing on low speed until just incorporated.

Step 6: Bake - Spoon the batter into lined cupcake tins, filling each well about two-thirds full. Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Step 7: Cool - Once baked, remove the cupcakes from the oven and let them cool in the pan for about 10 minutes before transferring to a wire rack.

Step 8: Make Buttercream - Beat 1.5 cups of room-temperature unsalted butter on high speed for about 10 minutes until pale and fluffy. Gradually add 1.35 lbs of powdered sugar and a pinch of salt, mixing on low until incorporated, then add in 3 tbsp of heavy cream.

Step 9: Decorate - Once cool, hollow out each cupcake's center, fill with chilled vanilla pudding, pipe on buttercream, and top with crumbled Nilla wafers.