Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan or line it with parchment paper, leaving an overhang on the long sides for easy removal.
Step 2: Wash your fresh rhubarb stalks thoroughly. Trim off the leafy ends and any tough bottom parts. Chop the rhubarb into 1/2-inch pieces. You'll need about 2 cups. Set aside.
Step 3: In a small bowl, combine the all-purpose flour, granulated sugar, and cold unsalted butter (cut into small pieces) for the streusel. Use your fingertips or a pastry blender to mix until the mixture resembles coarse crumbs. Set aside.
Step 4: In a large bowl, whisk together the 2 cups all-purpose flour, baking powder, baking soda, ground cinnamon, and salt. Make sure there are no lumps.
Step 5: In a separate medium bowl, whisk together the 1 cup granulated sugar, eggs, vegetable oil, vanilla extract, and sour cream until well combined and smooth.
Step 6: Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are fine.
Step 7: Gently fold in the chopped rhubarb pieces. Distribute them evenly throughout the batter.
Step 8: Pour the batter into your prepared loaf pan, spreading it evenly. Sprinkle the streusel topping generously over the top of the batter.
Step 9: Bake for 55-65 minutes, or until a wooden skewer inserted into the center of the loaf comes out clean. If the topping starts to brown too quickly, you can lightly tent the loaf with aluminum foil.
Step 10: Once baked, remove the loaf pan from the oven and let it cool in the pan on a wire rack for 10-15 minutes. Then, using the parchment paper overhang, carefully lift the rhubarb bread out of the pan and transfer it directly to the wire rack to cool completely before slicing.