Step 1: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease it thoroughly. This ensures your rhubarb muffins won't stick and makes for easy cleanup.
Step 2: Make the streusel topping. In a small bowl, combine the all-purpose flour, granulated sugar, and cold unsalted butter (cut into small pieces). Use your fingertips, a fork, or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Set aside.
Step 3: Prepare the dry ingredients for the muffins. In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, and salt. Make sure there are no lumps and all ingredients are evenly distributed.
Step 4: Prepare the wet ingredients. In a separate medium bowl, whisk together the buttermilk, melted unsalted butter, large egg, and vanilla extract until well combined.
Step 5: Combine wet and dry ingredients. Pour the wet ingredients into the dry ingredients. Using a spatula or a wooden spoon, mix until just combined. The key here is to avoid overmixing! A few lumps are perfectly fine; overmixing develops gluten, which can lead to tough muffins.
Step 6: Gently fold in the rhubarb. Add the chopped fresh rhubarb to the batter. Using a spatula, gently fold the rhubarb into the batter until it's just evenly distributed. Be careful not to mash the rhubarb pieces.
Step 7: Fill the muffin cups and add streusel. Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full. Generously sprinkle the prepared streusel topping over the top of each muffin.
Step 8: Bake your rhubarb muffins. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the tops are golden brown and a wooden skewer inserted into the center of a muffin comes out clean. Baking time can vary, so keep an eye on them.
Step 9: Cool and enjoy. Once baked, remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. Allowing them to cool slightly in the tin helps them set. These rhubarb muffins are best enjoyed warm or at room temperature.