In a large bowl, stir together cold water, condensed milk, and vanilla pudding mix until smooth and combined. Chill this in the fridge while you do the next steps or overnight.
In a large mixing bowl, use an electric hand mixer to whip heavy cream until it becomes stiff peaks.
Add the pudding to the whipped cream, half at a time, folding with a spatula until no streaks remain. Transfer this pudding mixture to a piping bag and snip the tip off.
In your small serving jars, pipe in a layer of pudding at the bottom. Arrange a layer of vanilla wafers and then banana slices on top. Top with another layer of pudding. Repeat with as many layers as you want until you've filled the jar.
Chill banana pudding in the fridge for at least 3-4 hours for the wafers to soften but serve on the same day before the bananas brown.