Preheat oven to 350 degrees. Spray a 9-inch round cake pan with cooking spray and set aside.
In the bowl of a stand mixer, whip together the butter and 1 cup of the sugar until pale and fluffy, 3 to 4 minutes.
Add the eggs, one at a time and beat well after each addition. Beat in the vanilla extract and sour cream until smooth.
In a medium bowl, whisk together the flour, baking powder, cardamom and salt. Add the flour mixture to the butter mixture. Start the mixer on low speed, scraping down the bowl as needed to incorporate the flour. Mix on a higher speed at the end to make a smooth batter with no streaks of flour remaining.
Fold in the sliced rhubarb by hand with a rubber spatula or wooden spoon. Spread the thick cake batter evenly into the prepared cake pan.
Sprinkle the remaining 1/4 cup of sugar evenly over the top of the cake.
Bake until a wooden pick inserted in the center comes out clean, 35-40 minutes.
Let the cake cool completely in the pan and slice and serve the cake from the pan.