1. Preheat Oven and Prepare Muffin Tin:
- Preheat your oven to 375°F (190°C).
- Line a standard 12-cup muffin tin with paper liners or grease and flour the cups well. Set aside.
2. Make the Streusel Topping (If Using):
- In a small bowl, combine the ½ cup all-purpose flour, ¼ cup packed brown sugar, and ½ teaspoon cinnamon. Whisk together.
- Add the ¼ cup of cold, cubed butter to the dry ingredients.
- Using a pastry cutter, a fork, or your fingertips, cut or rub the butter into the flour mixture until it resembles coarse crumbs with some larger pieces remaining. Don’t overwork it into a paste.
- Set the streusel mixture aside (you can pop it in the fridge to keep the butter cold if your kitchen is warm).
3. Mix the Dry Muffin Ingredients:
- In a large mixing bowl, whisk together the 1 ½ cups all-purpose flour, ¾ cup granulated sugar, ¾ cup packed brown sugar, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, and 1 teaspoon cinnamon until well combined.
4. Mix the Wet Muffin Ingredients:
- In a separate medium bowl, whisk together the melted (and slightly cooled) butter, the eggs, the buttermilk (or milk/vinegar mixture), and the vanilla extract until smooth.
5. Combine Wet and Dry & Fold in Rhubarb:
- Pour the wet ingredients into the large bowl containing the dry ingredients.
- Using a rubber spatula or wooden spoon, gently fold and stir the ingredients together just until combined. Do not overmix; a few lumps are perfectly okay. Overmixing leads to tough muffins.
- Add the diced fresh rhubarb to the batter. Gently fold it in with just a few strokes until evenly distributed.
6. Fill Muffin Cups and Add Streusel:
- Spoon the batter evenly into the prepared muffin cups, filling each cup about three-quarters (¾) full. An ice cream scoop can help portion evenly.
- If using the streusel topping, sprinkle it generously over the top of the batter in each muffin cup.
7. Bake the Muffins:
- Place the muffin tin in the preheated oven.
- Bake for 18-22 minutes, or until the tops are golden brown, spring back lightly when touched, and a toothpick inserted into the center of a muffin (avoiding rhubarb pieces) comes out clean.
8. Cool:
- Let the muffins cool in the muffin tin on a wire rack for about 5-10 minutes before carefully transferring them directly onto the wire rack to cool completely.
- Serve warm or at room temperature.