Position a rack in the center of the oven and preheat the oven to 350 degrees F.
Lightly butter 4 (4 by 1-inch) baking pans and dust with flour, tapping put any excess. Set aside.
In a medium bowl, whisk to combine the flour, baking powder, and salt; set aside. In a separate bowl, stir together the vanilla and the creme fraiche; set aside.
Melt 2 tablespoons of butter in a heavy skillet. Add the brown sugar and bourbon and cook over medium heat, stirring with a wooden spoon, until the sugar melts. Stir in the pecans to coat with the melted sugar mixture; remove from heat. Divide the mixture evenly between the prepared pans, working quickly to get it to the edges of the pans before it sets (cooked sugar cools rapidly). Arrange the rhubarb in circles over the sugar, and set aside while you make the batter.
Place the remaining 4 tablespoons butter and the granulated sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed until the mixture is smooth and creamy, scraping down the sides of the bowl with a rubber spatula, as needed, 3 to 4 minutes. The butter and sugar must be beaten until they are light, fluffy, and pale. Reduce the speed to medium and add the eggs 1 at a time, beating well after each addition.
Using a rubber spatula, carefully fold in the dry ingredients and the creme fraiche mixture alternately, beginning and ending with the dry ingredients. The batter will be thick.
Spoon the batter over the rhubarb and smooth the tops by rotating the pans while you run a rubber spatula over the batter. Transfer the pans to a baking sheet and bake until a toothpick inserted in the center of a cake comes out clean, 20 to 25 minutes. As soon as the cakes are removed from the oven, turn them out of their pans onto a rack. Serve with whipped cream, if desired. The cakes can be kept wrapped in plastic at room temperature overnight.
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