To make the vanilla kefir pudding: In a medium saucepan, set over medium heat, combine sugar with 3/4 cup of the whole milk and salt. Bring to a gentle simmer, stirring occasionally, until sugar has dissolved.
Meanwhile, combine the remaining 1/4 cup milk with the cornstarch. Add the milk and cornstarch mixture to the saucepan with the milk and sugar mixture. Whisk and cook until 30 seconds to a minute, until it thickens.
Run through a sieve into a bowl. Slowly whisk in the vanilla extract and kefir until smooth. Place a sheet of plastic wrap onto the surface of the pudding and transfer to the fridge to cool, at least 2 hours.
To assemble the banana pudding: Using a hand mixer, beat the heavy cream and sifted powdered sugar together until very soft peaks form, about 1 to 2 minutes.
To assemble the pudding: Add a few tablespoons of pudding to each glass. Top with a few slices of banana, broken up wafers, a few tablespoons of whipped cream. And then more banana, wafers, pudding and then more bananas, wafers and lastly, finish with a liberal dollop of whipped cream. Of course, this doesn’t have to be in a crazy order—do what you like! I finished with a few crumbs of wafers.