• Adjust oven rack to lower-middle position and heat oven to 350 degrees. Grease 8-inch square baking pan, line bottom with parchment paper, and grease parchment.

Prepare Streusel

• Stir all ingredients in medium bowl until well combined. Set aside.

Prepare the Rhubarb

• Whisk sugar, cornstarch, and lemon zest together in large bowl. Add rhubarb and stir well to coat. Drizzle with melted butter and stir to incorporate. Transfer rhubarb mixture to prepared pan and press rhubarb pieces into bottom of pan, making sure there are no large gaps (pieces may not fit in single layer).

Prepare the Cake

• Whisk flour, cardamom, baking powder, and salt together in medium bowl; set aside. Whisk sugar and eggs in large bowl until thick and homogeneous, about 45 seconds. Whisk in melted butter until combined. Add sour cream, lemon zest and juice, and vanilla; whisk until combined. Add flour mixture and whisk until just combined.
• Pour batter into pan and spread evenly over rhubarb mixture. Break up streusel with your hands and sprinkle in even layer over batter.

  Bake until cake is golden brown and toothpick inserted in center comes out clean, 45 to 50 minutes.

Finish the Dessert

• Transfer pan to wire rack and let cool for 20 minutes. Run knife around edges of pan to loosen cake, then invert onto serving platter. Let cool for about 10 minutes.
• Microwave jelly in small bowl until fluid, about 20 seconds. Using pastry brush, gently dab jelly over rhubarb topping. Serve warm or at room temperature.