Preheat oven to 325F. Grease and 8-inch springform pan and line the bottom with a round of parchment paper.
In a large bowl with an electric mixer, or in a the bowl of a stand mixer, fitted with the paddle attachment, beat the butter, brown sugar and vanilla until light and fluffy, about 2-3 minutes. Add the eggs, one at a time, beating well after each addition and scraping down the sides of the bowl several times, as needed. Add the flour and mix, just until combined.
Remove 1/4 cup of the batter to a small bowl. Add the 7 Tbsp flour and 1 tsp cinnamon and stir together using a dinner knife, until it becomes a crumbly mixture. Set aside.
Fold the rhubarb into the rest of the cake batter, then spoon the batter into the prepared springform pan, spread to the sides and level the top. Evenly scatter the crumble mixture over-top of the batter, then sprinkle the top with 1 Tbsp of light brown sugar (or white sugar).
Bake in preheated oven for about 1 hour and 15 minutes to 1 hour and 30 minutes, but check and test regularly after the 1 hour mark and cover the top loosely with a sheet of aluminum foil if the top of the cake is browning too quickly. Bake until a tester comes out clean, as long as that takes. (*If starting with frozen rhubarb, cooking time will be slightly longer).
Remove from oven and cool for 15 minutes in the tin, then remove from pan and cool completely on a cooling rack.