Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it well.

In a large bowl, whisk together the flour, baking soda, baking powder, and salt.

In a separate bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the eggs one at a time, then stir in the mashed bananas and vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Mix until just combined; do not overmix.

Gently fold in the chopped rhubarb.

For the streusel topping (if using): In a small bowl, combine the flour and brown sugar. Cut in the cold butter with a pastry blender or your fingertips until crumbly.

Spoon the batter evenly into the prepared muffin cups, filling each about two-thirds full. Sprinkle with streusel topping, if desired.

Bake for 20-25 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean.

Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.