• Preheat oven to 350 F.
• Line the bottom of the springform pan with parchment paper. Grease the side of the pan for easy removal and to prevent the cake from sticking to the sides.
• Chop the fresh rhubarb into 1/2 inch bite-sized pieces. Place them into a small bowl and set aside.

For the crumble

• Combine all the dry ingredients in a medium-sized bowl. Add the melted butter and mix well with a fork until crumbly. Place into the fridge until ready to use.

For the Rhubarb Cake Batter

• Use a hand mixer to cream butter and sugar until light and fluffy.
• Add egg, vanilla, sour cream, and sourdough discard. Mix until combined.
• Next, add baking powder, baking soda, and salt. Add flour. Mix until just combined, and you can't see any more dry bits of flour.
• Set 1/2 cup of rhubarb to the side. Fold the rest of the chopped rhubarb into the cake mixture with a spatula. (Note that this batter is on the thicker side.)
• Be sure not to overmix the batter; this can lead to a dry and tough crumb.
• Dump into the prepared springform pan and use a spatula to spread to the sides.
• Remove the crumble from the fridge and top the rhubarb cake batter with the crumble. Be sure to get all the edges of the pan for an even top layer. Add the 1/2 cup of chopped rhubarb, which was set aside, on top of the crumble for a pop of color.
• Bake on the middle rack for 45 to 50 minutes. The top should be light golden brown. A toothpick should come out clean, and the middle shouldn't be jiggly.
• Let the cake cool before slicing and serving. Enjoy with a dollop of whipped cream or some homemade vanilla ice cream!