• Preheat the oven to 325°F. Butter the sides and bottom of a 9-inch springform pan. Wrap 2 layers of foil under the pan and place it on a baking sheet.
• In a medium bowl, add the chopped rhubarb, cornstarch and sugar; toss to combine.
• For the sauce; in a small saucepan over medium heat, whisk together the brown sugar and butter. Continue whisking until the butter is melted and the sauce is smooth and bubbling; about 2 minutes.
• Combine the dry ingredients In a separate bowl, whisking the flour, baking powder, cinnamon, salt, and cardamom.
• With an electric or stand mixer, cream together 1 cup room temperature butter and 1 cup sugar, beating until it is light and fluffy; about 3 minutes. Add the eggs, vanilla, and sour cream to the bowl, and mix on low speed until just combined. Gradually add the flour mixture to the butter mixture and mix until smooth; scrape down the bowl as needed.

Assemble the Cake

• Pour the Brown Sugar Sauce into the bottom of the springform pan, then spoon in the rhubarb with its juices over the top. Then spoon in the cake batter so that it covers all of the rhubarb. Smooth the top with a spatula.
• Bake for 1 hour and 15 minutes, or until the top of the cake is firm and a toothpick inserted in the middle comes out mostly clean.
• Let the cake cool in the pan for 15 minutes then run a knife around the outside edges of the cake. Place a plate on top of the pan and turn it upside down. Then release the cake from the pan while it's still warm, otherwise, it will stick to the pan.