Cook the mixture over medium heat, whisking until it turns into a light and thickened custard. Remove the custard from the heat and whisk in the butter. Allow it to cool for about an hour.
Add the cooled custard to a large mixing bowl and fold in the whipped cream.
In a separate bowl, combine the pineapple chunks, mandarin oranges, grapes, mini marshmallows, and pecans.
Pour the cooled custard mixture over the combined fruit and toss to coat well. Slice the bananas and gently fold them into the fruit salad mixture.
Cover the freshly assembled fruit salad with plastic wrap and chill overnight in the refrigerator. Serve this overnight fruit salad chilled with optional garnish such as additional nuts, maraschino cherries, and coconut flakes.