Preheat Oven: Set oven to 400°F (200°C). Line a 12-cup muffin pan with paper liners or lightly grease.
Prepare Dry Ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
Prepare Wet Ingredients: In a large bowl, whisk together yogurt, milk, oils, eggs, brown sugar, granulated sugar, and vanilla until smooth.
Toss Rhubarb: Lightly toss diced rhubarb with 1–2 tsp flour to prevent sinking.
Combine Batter: Gently fold dry ingredients into wet mixture until just combined. Carefully fold in rhubarb. Avoid overmixing to keep muffins tender.
Fill Muffin Cups: Using a spoon or ⅓-cup measuring cup, evenly divide batter among the muffin cups.
Add Streusel (Optional): In a small bowl, mix sugar, cinnamon, and melted butter. Sprinkle about 1 tsp over each muffin.
Bake: Bake 20–25 minutes, or until tops are lightly golden and a toothpick inserted comes out clean.
Cool: Let muffins cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.