This variation on the all-American favorite, Pineapple Upside-Down Cake, uses rhubarb instead. The cake is light and moist with a tender crumb, and the sticky caramel topping perfectly sweetens the tartness of the fruit. The cake tastes best when served warm with a scoop of vanilla ice cream or a drizzle of Rhubarb Raspberry Sauce.
Position a rack in the center of the oven and preheat the oven to 350 degrees F. Butter an 8-inch round cake pan and line the base with a round of parchment paper. Butter the paper. Set the pan aside.
Place the 1/4 cup of butter in a small saucepan, and melt over medium heat. Stir in the brown sugar and honey, and bring the mixture just to a boil. Simmer for 2 minutes or until bubbly, then pour the caramel into the prepared pan, spreading it evenly across the bottom. Press the rhubarb slices into the caramel, rounded side down, working in concentric circles. If there are gaps, either fill these spaces with smaller pieces of rhubarb or leave empty. Set aside at room temperature.
Combine the eggs and sugar in the bowl of a mixer and beat to blend. Add the sour cream, vanilla, and butter, and beat until smooth.
In a separate bowl, combine the flour, baking powder, baking soda, and salt, and whisk to blend. Add the dry mixture to the batter and mix at slow speed just until combined. Spoon the batter over the fruit carefully. Bake until the cake is golden and a toothpick or skewer inserted into the center of cake comes out clean, 45 to 55 minutes.
Cool the cake on a wire rack for 5 minutes, then turn out onto a serving platter. Be sure to remove the cake from the pan before it cools, or the caramel may harden and stick to the pan. If any pieces of rhubarb remain in the pan, carefully replace them. Slice the cake into wedges and serve warm.