Line 2 baking sheets with parchment paper. Place the gold sparkling sugar in a medium bowl.
In a large bowl, combine the brown sugar, butter, vanilla, salt and baking powder and beat with an electric mixer on medium-low speed until creamy, about 1 minute (see Baker’s Note). Add the egg and beat until smooth. Add the flour and beat on low speed until just combined.
Using a half-ounce ice cream scoop or 1 tablespoon, portion the dough and roll into balls then coat in the gold sugar. Arrange the balls 2 inches apart on the prepared baking sheets. Using the end of a wooden spoon (half-inch diameter) or your index finger, press each dough ball straight down in the center to create a deep divot. Refrigerate the dough on the baking sheets for 30 minutes.
Heat the oven to 375 degrees. Bake the chilled cookies, rotating the baking sheets from front to back and top to bottom halfway through, until light golden brown on the bottom, 10 to 12 minutes. Let the cookies cool for 1 minute on the baking sheets, and then transfer to a wire rack to cool completely.
Make the filling: Melt the butter in a small saucepan over medium-high heat. Stir in the brown sugar and cream, bring to a simmer and cook, stirring, until bubbly and thick, 4 to 5 minutes. Remove the pan from heat and stir in the whiskey, vanilla and kosher salt.
While the filling is still hot, use a quarter-teaspoon to fill the divot in each cookie with the warm butterscotch sauce. Sprinkle with flaky sea salt and gold dragées, if you like, while warm, then let cool to set, at least 30 minutes.
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