Step 1: Preheat oven to 375°F (190°C). Grease and flour a 9×13 inch baking pan or line with parchment paper.
Step 2: In a medium bowl, whisk together 2 cups of all-purpose flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream together 1 cup softened unsalted butter and 1 1/2 cups granulated sugar with an electric mixer until light and fluffy, about 3-5 minutes.
Step 4: Beat in eggs one at a time, ensuring each is fully incorporated. Stir in vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.
Step 6: Gently fold in the chopped fresh rhubarb. Spread the batter evenly into the prepared baking pan. Set aside.
Step 7: In a separate medium bowl, combine the remaining 1/2 cup all-purpose flour, light brown sugar, 1/4 cup granulated sugar, and ground cinnamon. Whisk together.
Step 8: Add the cold, cubed 1/2 cup unsalted butter to the dry topping ingredients.
Step 9: Using a pastry blender, your fingertips, or a fork, cut the butter into the flour mixture until coarse crumbs form, with some pea-sized pieces remaining.
Step 10: Evenly sprinkle the crumb topping over the rhubarb cake batter in the pan.
Step 11: Bake for 45-55 minutes, or until a wooden skewer inserted into the center comes out clean and the topping is golden brown.
Step 12: Remove from oven and let cool in the pan on a wire rack for at least 30 minutes before slicing and serving.