Make the crumb topping: In a medium bowl, stir together the flour, sugars and cinnamon. Using your fingers, rub in the butter, squeezing to form clumps. Place bowl in freezer until needed.
Preheat oven to 350 F.
Make the cake: In a large bowl with an electric mixer or the bowl of a stand mixer fitted with a paddle attachment, beat the butter with 2/3 cup white sugar until combine. Beat in eggs, one at a time. Beat in vanilla. Scrape down bowl.
In a separate bowl, whisk together the flour, baking powder, baking soda and salt. With mixer on low, stir in flour alternately with sour cream, making 3 additions of the flour and 2 additions of the sour cream.
Spread batter in to a parchment-lined 9x9-inch square baking pan. Sprinkle batter with 1/2 of the crumb mixture. Top with rhubarb. Sprinkle 1 Tbsp. white sugar over the rhubarb, then top with remaining crumb mixture.
Bake in the centre of oven until tester comes out clean, about 55-65 minutes.
Be sure to read the "Cook's Notes" in the original post, for more tips, options, substitutions and variations for this recipe!