Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 9x13 inch baking pan (or a 9-inch square pan for a thicker cake, adjusting bake time as needed). You can also line it with parchment paper for easier removal.
Step 2: Prepare the Streusel Topping: In a medium bowl, combine the cold butter cubes, granulated sugar, packed light brown sugar, cinnamon, and 1 1/2 cups all-purpose flour. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Set aside in the refrigerator while you prepare the cake batter to keep it cold and crumbly.
Step 3: Prepare the Rhubarb: In a separate small bowl, toss the chopped rhubarb with 1 tablespoon of granulated sugar and 1 tablespoon of all-purpose flour. This light coating helps prevent the rhubarb from sinking to the bottom of the cake and ensures it is evenly distributed.
Step 4: In a large mixing bowl, cream together the softened butter and 1 1/2 cups granulated sugar with an electric mixer until light and fluffy (3-5 minutes). Scrape down the sides of the bowl as needed to ensure everything is well combined.
Step 5: Beat in the 2 large eggs, one at a time, mixing well after each addition until fully incorporated. Stir in the vanilla extract.
Step 6: In another medium bowl, whisk together the remaining 2 1/4 cups all-purpose flour, baking soda, baking powder, and salt. This ensures all dry leavening agents and salt are evenly distributed throughout the mixture.
Step 7: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix on low speed until just combined. Be careful not to overmix, as overdeveloping the gluten can lead to a tough cake; a slightly lumpy batter is perfectly fine.
Step 8: Gently fold in the sugared rhubarb pieces into the cake batter using a spatula. Mix just until the rhubarb is evenly distributed throughout the batter.
Step 9: Pour the cake batter evenly into your prepared baking pan. Remove the chilled streusel topping from the refrigerator and crumble it evenly over the top of the batter, ensuring good coverage.
Step 10: Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean and the streusel is golden brown and bubbly. Baking times may vary, so check for doneness starting around 45 minutes. If the streusel browns too quickly, you can loosely tent the cake with aluminum foil.
Step 11: Remove the cake from the oven and let it cool completely on a wire rack before slicing and serving. Cooling is essential for the cake to set properly and allows the flavors to meld beautifully.