Step 1: Prepare the streusel topping. In a small bowl, combine 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 2 tablespoons turbinado sugar, and 1/4 cup cold unsalted butter (cut into small pieces). Use your fingertips, a pastry blender, or a fork to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Set aside.
Step 2: Preheat your oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease it thoroughly.
Step 3: In a large bowl, whisk together the dry ingredients for the muffins: 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.
Step 4: In a separate medium bowl, whisk together the wet ingredients: 1 cup buttermilk, 2 large eggs, 1/2 cup melted and cooled unsalted butter, and 1 teaspoon vanilla extract. Whisk until smooth.
Step 5: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are fine.
Step 6: Gently fold in the 1 1/2 cups diced fresh rhubarb, distributing it evenly throughout the batter.
Step 7: Divide the muffin batter evenly among the 12 prepared muffin cups, filling each about two-thirds full.
Step 8: Generously sprinkle the prepared streusel topping over the top of each muffin.
Step 9: Bake for 5 minutes at 400°F (200°C). Then, reduce the oven temperature to 375°F (190°C) and continue baking for another 20-25 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean and the tops are golden brown.
Step 10: Remove the muffins from the oven and let them cool in the muffin tin for about 5 minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.