Preheat the oven to 375°f / 190°c. Line 1-2 12 hole muffin pans with paper liners (see notes on spacing out muffins)

RHUBARB STREUSEL

1. Place the sugar into a small bowl. Add the orange zest and vanilla if using, and rub in using your fingers. Add in the flour and salt and mix to combine, then add in the butter and rub in using your fingers until the butter is incorporated and the mixture is crumbly.
2. Add in the sliced rhubarb and mix to combine. Set aside.

MUFFINS

1. Whisk together the flour, sugar, baking powder, baking soda, and salt in a large bowl. In a medium bowl, whisk together the egg, yoghurt, butter, milk, and vanilla bean paste.
2. Add the wet ingredients into the dry ingredients and fold lightly with a spatula to only just combine, then add the chopped rhubarb in and finish folding in the dry ingredients along with the rhubarb (it is important not to over mix muffins).
3. Using a cookie scoop or a cup measure, measure 1/4 cup of batter into each muffin cup, smoothing down lightly with a spoon if needed. Top each with about a Tablespoon of the rhubarb streusel, spreading over the surface of the muffin with your fingers.
4. Bake the muffins for 30 to 35 minutes, or until the centres spring back when lightly pressed and a skewer inserted comes out clean. Transfer the pan to a wire rack and allow to cool for 5 minutes before removing from the pan and allowing to cool completely.
5. Store leftovers in an airtight container at room temperature.