In a medium saucepan over medium-low heat, combine egg yolks, reserved pineapple juice, sugar, vinegar, salt, and butter.
Stir constantly with a silicone spatula until thickened. Make sure to scrape the sides and bottom of the pan to avoid burning the custard. It will take 10-15 minutes to cook the custard, but don’t be tempted to increase the cooking temperature to speed the process up. If you let the custard boil, the eggs will scramble and the custard will curdle.
Cook the custard until it reaches nappe (i.e., when the curd coats the back of a spoon and maintains a line when a finger is drawn across the spoon instead of dripping down to fill in the line) or approximately 180°F/83°C.
Pour the custard immediately into a large bowl so the custard stops cooking. Allow the custard to cool to room temperature. Add fruit, marshmallows, and almonds and stir gently to combine.
Whip the heavy cream to stiff peaks and fold into the fruit mixture. The salad can be eaten immediately, but it is better if you cover the bowl with plastic wrap and refrigerate for 24 hours. Chilling the 24-Hour Salad allows the flavors to blend.