Preheat your oven to 350 degrees. Butter your nine-inch round cake pan. Line the bottom with a piece of parchment paper.
Trim your rhubarb stalks into even pieces. Lay them across the bottom of the pan. Make sure they cover the surface in one layer.
Sprinkle a little sugar over the raw rhubarb. This helps the fruit caramelize while it bakes. Set the pan aside for a few minutes.
Whisk your flour, baking powder, and salt together. Ensure there are no lumps in the dry mixture. This creates a very smooth cake batter.
Cream the butter and sugar in your mixer. Beat them until the mixture looks pale and fluffy. This step adds air to the cake.
Add the eggs one at a time. Mix well after you add each egg. Pour in the vanilla extract and mix again.
Fold in the dry ingredients and sour cream. Alternate between the two for a smooth texture. Do not overmix the batter at this stage.
Spread the batter over the rhubarb in the pan. Use a spatula to smooth out the top. Ensure the fruit is completely covered by batter.
Bake the cake for about forty-five minutes. A toothpick should come out clean from the center. The top will look golden and firm.
Cool the cake in the pan for ten minutes. Run a knife around the edges to loosen it. Invert it carefully onto a serving plate.