For the pretzel rods: Place four 10-ounce glasses in the freezer and line a baking sheet with wax paper. In a short, very narrow glass or tumbler, stir together the chocolate and vegetable oil. Dip the pretzel rods in the chocolate, tilting the glass and twirling the rods to coat. Transfer to the prepared baking sheet and sprinkle with cayenne. Place in the refrigerator until set, at least 20 minutes.
For the caramel sauce: Melt the brown sugar and butter in a small saucepan over medium-high heat, whisking until the sugar melts. Add the half-and-half and vanilla and bring to a boil; cool until thickened, 3 minutes. Season with some salt and let cool.
For the milkshakes: In the bowl of a blender, combine the ice cream, bourbon, vanilla and a sprinkle of Maldon salt. Blend until smooth and frothy, adding milk if necessary to thin. Pour into the chilled glasses.
Drizzle each milkshake with the caramel sauce, sprinkle with some Maldon, and garnish with a pretzel rod. Serve immediately.