PreparationBourbon sugar: Preheat oven to 150° or as low as yours will go. Scrape vanilla seeds into a small bowl; stir in demerara sugar and bourbon. Spread on a parchment-lined baking sheet and let dry out in oven overnight, leaving door slightly ajar. Mixture should feel like demerara sugar again in the morning. Fudge: Line an 8x8" baking dish with parchment paper, leaving a generous overhang on 2 sides; lightly coat with nonstick spray. Heat chocolates in a heatproof bowl set over a saucepan of simmering water (do not let bowl touch water), stirring occasionally, until almost completely melted. Set aside. Combine condensed milk, butter, bourbon, corn syrup, and kosher salt in a small saucepan; scrape in vanilla seeds and add pod. Heat over medium until barely hot (same as the chocolate). Gently stir one quarter of milk mixture into chocolate with a rubber spatula. Add another quarter of milk mixture, stirring to incorporate (it might look broken and greasy; don't worry, it will come back together). Continue with remaining milk mixture in 2 additions, stirring vigorously until fudge is very shiny and almost elastic, about 5 minutes. Scrape into prepared pan and smooth top. Sprinkle with bourbon sugar and sea salt. Let cool, then cover and chill at least 4 hours. Turn out fudge onto a cutting board and slice into 2x3/4" rectangles, 2" squares, or triangles. Do ahead:Store bourbon sugar airtight at room temperature up to 2 months.Fudge can be made 1 week ahead. Wrap tightly and chill.