Blend 1/2 cup of the sugar and the cornstarch in a medium pot. Stir in milk. Bring to a simmer, stirring often. Whisk 1 cup of the mixture into beaten egg yolks. Stir egg mixture back into pot. Cook 4 to 5 minutes, until it reaches 160°, thickens and begins to bubble. Remove from heat; stir in vanilla and butter. Cool completely.
In the bottom of each jar, place 3 vanilla wafers. Cover with a layer of bananas. Spoon a scant 1/4 cup of the pudding on top of each. Repeat layering. Secure lids; refrigerate at least 3 hours or overnight.
Just before serving, combine heavy cream and remaining 1 tbsp sugar in a bowl. Using a hand mixer, whisk until stiff peaks form. Spoon over each dessert.