Preparations: Preheat the oven to 375°F (180°C). Grease and flour the springform pan, then gently tap out the excess flour over the sink.
Rhubarb and apples: Wash, dry, and cut the rhubarb into small pieces (about ½ inch / 1 cm). Place them in a large bowl. Peel, core, and chop the apples to about the same size as the rhubarb. Add them to the rhubarb, stir in the sugar, and set the bowl aside while you prepare the batter.12.5 oz rhubarb + 2 small apples + 2 tablespoons granulated sugar
Beat wet ingredients: Beat the sugar and soft butter at high speed until light and fluffy, about 3-4 minutes. Scrape down the sides of the bowl with a rubber spatula. Add the eggs one at a time, beating for about 30 seconds after each. Add the vanilla extract and stir for another minute.¾ cup granulated sugar + ¾ cup unsalted butter + 3 large eggs + 1 teaspoon vanilla extract
Add dry ingredients: In another bowl, mix the flour, baking powder, and salt. Add this mixture to the wet ingredients and stir briefly at low speed until just combined.1 ½ cups all-purpose flour + 2 teaspoons baking powder + ¼ teaspoon fine sea salt
Assemble the cake: Transfer the batter to the prepared pan. Evenly spread the rhubarb and apple mixture over the top.
Bake the apple and rhubarb cake for about 55 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely in the pan. Then transfer it to a serving platter and, if desired, sprinkle with confectioner’s sugar.a little confectioner’s sugar
Notes
Rhubarb: As shown in the pictures, I had lots of very thin rhubarb stalks. Regular-sized (thicker) stalks work perfectly too. If they are very thick, cut them in half lengthwise first, then chop into smaller pieces.
Apples: Use sweet-tart apples that hold their shape while baking. I used Pink Lady, but Braeburn or Granny Smith are great options as well.