Upside-down cakes may be old-fashioned, but in my world, they never go out of style. The leggy stalks of rhubarb make it extra elegant, and quite a showstopper, but no one needs to know how easy this cake is to make!
Any remaining rhubarbdiced, up to 1 cup (optional)
Make the Topping
Preheat the oven to 375°F with a rack placed in the center of the oven.
Following the shape of your 8- or 9-inch cake pan, cut the rhubarb to fit. I like to line up the rhubarb in a row, place the pan on top and trim. Fine tune the cuts so that they snugly fit in the pan. Set the rhubarb aside.
Lightly butter or grease the cake pan.
Melt the butter, then mix with the brown sugar, cinnamon and ginger in a bowl. Pour the sugar mixture into the prepared cake pan, then line up the rhubarb.
Make the Cake
Using an stand mixer or hand mixer, beat the butter until soft. Add the sugar and egg and beat until fluffy.
Add the egg, salt, baking powder and vanilla extract and beat until smooth, scraping down the sides of the bowl.
Working in batches, add the flour and the buttermilk, and mix, alternating and ending with the flour. Do not over mix.
If you have any remaining rhubarb and wish to use it in the cake batter, gently fold it into the batter.
Spread the batter over the rhubarb topping, and bake in the oven for about 30-35 minutes, or until the cake is firm to the touch and an inserted toothpick comes out clean.
Let the cake rest for 3 minutes.
Place a plate on top of the cake pan and invert. Wait about 30 seconds, then lift the pan to release the cake.
Serve the cake warm or at room temperature.
Upside-down cake is most delicious when served warm. If you're making the cake in advance, let the cake cool completely and store, covered, in the refrigerator. Reheat before serving.
If you have any remaining rhubarb and wish to use it in the cake batter, it adds another layer of rhubarb flavor, similar to my favorite rhubarb cake.