Preheat the oven to 350 °F (175 °C). Butter and flour a 9x9-inch (23x23-cm) square pan (preferably non-reactive). Set aside.
Slice rhubarb in half, lengthwise (or even in quarters for thicker stalks of rhubarb) and then into 2 ½" pieces. Select about 36 of the prettiest and set them aside for the top of the cake. Chop the rest into ½" pieces. Set aside.
In a small food processor (like this KitchenAid on Amazon), pulse together the flour, pumpkin seeds, baking powder, and salt, until the seeds are finely ground and the dry ingredients are evenly mixed.
In the bowl of a stand mixer fitted with the paddle attachment (like this KitchenAid on Amazon), cream together the butter and sugar, then beat them together on medium-high speed for a full 4 minutes, until light and fluffy. Scrape down the bowl as needed with a spatula. Add the eggs, one at a time, beating well between each addition. Then add the vanilla, and beat the mixture again for another 4 minutes on medium-high until it's very pale, light (like soft whipped cream).
With the mixer on low, add half the dry ingredients, mix them in, then add the sour cream. Scrape down the bowl again, then add the rest of the dry ingredients, and stir just to combine. Take the bowl off the stand, then fold in the ½" rhubarb pieces with a spatula. The batter will be very thick.
Spoon the batter into the prepared pan, smearing and pressing it into the corners and edges of the pan. Then arrange the reserved sticks of rhubarb on the top into a pattern (see picture). Sprinkle with 2 tablespoon granulated sugar. If you are using a pan with a removable bottom, place it on a baking sheet.
Bake the cake for at least 80 minutes, rotating after about an hour of baking. Bake the cake until the edges are nicely browned, and a skewer inserted into the middle comes out clean.
Let cool before serving. Unmold after about 20 minutes if you are using a removable bottom tart pan.