Scrape seeds from one of the vanilla beans. In a medium saucepan, combine seeds, scraped bean, granulated sugar, corn starch, salt, and milk. Bring to a simmer over medium heat, stirring to dissolve sugar. In a separate bowl, whisk together egg yolks and brown sugar. When milk mixture starts to simmer, whisk about 1/3 of the hot liquid into the yolk mixture to temper it. Pour everything back into saucepan and continue to cook over medium heat for 4 to 5 min., stirring, until mixture starts to thicken. Add 2 Tbsp. of the bourbon and continue to cook, whisking continuously, until mixture comes to a boil. Once boiling, continue cooking and whisking for 1 more min. Remove from heat, transfer to a bowl, and cover with plastic wrap pressed directly on top of pudding to prevent a skin from forming. Refrigerate for at least 6 hours or up to overnight.
When ready to assemble, preheat a well-oiled grill to medium-high. Slice bananas in half lengthwise, leaving peel on. Generously rub Vanilla Bean Sugar on each banana half. Grill bananas, sugar side down, for about 5 to 7 min. until sugar begins to caramelize; move bananas away from direct heat if sugar starts to burn. Remove bananas from grill, let cool, then remove peel and slice into 1/2-inch chunks.
Meanwhile, scrape seeds from remaining vanilla bean and add to mixing bowl with cream and remaining 2 Tbsp. bourbon. Whip until stiff peaks form.
To assemble, create the following layers a couple of times in parfait glasses or small jars: vanilla wafers, pudding, whipped cream, banana slices. Top each parfait with a dollop of whipped cream and garnish with vanilla wafers and/or banana slices. Cover and refrigerate until ready to serve.