Prepare the Baking Dish: Line a 9x9-inch baking dish with parchment paper, making sure it extends over all four sides. Lightly spray the paper with nonstick cooking spray and set aside.
Mix Banana Flavoring and Color: In a small bowl, combine banana flavoring and yellow food color gel. Stir to combine and set aside.
Prepare the Fudge Base: In a heavy-bottomed saucepan, add butter, half-and-half, heavy cream, and vanilla. Stir continuously over medium heat until butter is melted.
Add Sugar: Once the butter is melted, add sugar and stir constantly as the mixture comes to a simmer (about 6-7 minutes). Let it simmer for an additional 3 minutes.
Melt the White Chocolate Chips: Remove the pan from heat, add the white baking chips, and cover. Let it rest for 3-4 minutes, then stir until the chips are melted.
Incorporate Marshmallow Fluff: Add marshmallow fluff and stir until fully combined.
Add Banana Mixture: Stir in the banana flavoring and yellow food gel mixture until evenly combined.
Spread the Fudge: Pour the fudge mixture into the prepared baking dish. Use a spatula to spread it evenly. Optionally, sprinkle crushed Nilla wafers over the top.
Chill and Set: Cover with aluminum foil and allow to sit at room temperature for 30 minutes, then chill in the refrigerator for at least 4 hours.
Slice and Serve: Once set, lift the fudge from the dish using the parchment paper and slice into 25 pieces. Store refrigerated until serving.
