Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with liners or grease lightly.
In a medium bowl, combine all streusel topping ingredients. Use fingers or a pastry cutter to form coarse crumbs. Refrigerate until needed.
In a large bowl, whisk together flour, baking powder, baking soda, and salt.
In another bowl, whisk melted butter and sugar until smooth. Add eggs one at a time, then vanilla and buttermilk.
Pour the wet ingredients into the dry, mixing until just combined.
Toss diced rhubarb with 1 tbsp flour, then fold into the batter.
Divide batter evenly among muffin cups, filling ¾ full. Top generously with streusel.
Bake for 20–25 minutes until golden and a toothpick comes out clean. Cool in pan 5 minutes, then transfer to wire rack.