Source: Johanne Killeen, Baking with Julia

1 2/3 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon salt, preferably kosher or fine sea salt
2 teaspoons pure vanilla extract
1 cup creme fraiche, homemade or store-bought, or sour cream
6 ounces unsalted butter, at room temperature
1/2 cup dark brown sugar (lightly packed)
1 tablespoon bourbon
2 tablespoons chopped pecans
6 to 7 stalks (12 ounces) fresh rhubarb, trimmed and cut into 1/4-inch-thick slices
1 cup granulated sugar
2 large eggs, at room temperature
Melted butter, for greasing the pans
Lightly sweetened whipped cream or creme fraiche for serving

Position a rack in the center of the oven and preheat the oven to 350 degrees F. Brush the insides of 8 mini or baby cake pans, each 4 inches across and 1 inch deep, with a light coating of melted butter, dust with flour, and tap out the excess.

Whisk or stir the flour, baking powder, and salt together just to blend; reserve.

In a separate bowl, stir the vanilla extract into the creme fraiche and set aside until needed.

Melt the 1/2 stick 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 caramel and turn off the heat. Divide the caramel evenly among the 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.

Put the remaining stick of butter and the granulated sugar in the bowl of a mixer fitted with the paddle attachment, or use a hand-held mixer, and 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. The butter and sugar must be beaten until they are light, fluffy, and pale, so don't rush it - the process can take 3 to 4 minutes with a heavy-duty mixer and 6 to 8 minutes with a hand-held mixer. Reduce the speed to medium and add the eggs one at a time, beating well after each addition.

Working with a rubber spatula, carefully fold in the dry ingredients and the creme fraiche alternately - 3 additions of dry ingredients, 2 of creme fraiche. You'll end up with a thick batter.

Spoon the batter over rhubarb and smooth the tops by rotating the pans while you run a rubber spatula over the batter. Put the pans on a jellyroll pan and bake for 20 to 25 minutes, or until a toothpick inserted in the center of a cake comes out clean (test a couple of the cakes to be certain). 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.

Large Upside-down Cake: Make the caramel in a heavy 11- or 12-inch skillet--cast iron is ideal for this. Arrange rhubarb over the caramel, spoon in the batter, and bake the cake for 45 to 50 minutes.