For the Cake:

1. Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.
2. In a large bowl, mix together the vanilla cake mix, water, vegetable oil, eggs, and vanilla extract. Beat on medium speed until smooth and well combined (about 2 minutes).
3. Pour the batter into the prepared baking pan and spread it evenly.
4. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely in the pan.

For the Filling:

1. In a medium bowl, whip the heavy cream with an electric mixer until soft peaks form.
2. Add the powdered sugar, vanilla extract, and marshmallow fluff. Continue to whip until the mixture is smooth and stiff peaks form.
3. Once the cake has cooled, cut a rectangular hole in the center of the cake (about 1 inch deep and wide), leaving about 1 inch of cake around the edges.
4. Fill the center of the cake with the marshmallow filling, spreading it evenly.

For the Frosting:

1. In a separate bowl, beat the softened butter with an electric mixer until creamy.
2. Gradually add the powdered sugar and vanilla extract, mixing until smooth.
3. Add the heavy cream, a tablespoon at a time, until the frosting reaches your desired consistency.
4. Spread the frosting over the entire cake, smoothing it out to cover the filling.