In a small bowl, stir together flour, sugar, zest, and salt. Stir in butter until crumbly. Set aside.
Preheat oven to 400° F.
Place sugar and lavender in food processor; pulse 30 to 60 seconds (pulse longer for a more intense lavender flavor). Sift through strainer into medium bowl to remove any large pieces of lavender.
Add flour, baking powder, baking soda and salt until well-combined.
In a small, whisk together melted and slightly cooled butter, yogurt, zest, egg and vanilla. Make well in center of dry mixture; pour in wet mixture. Stir until just combined. Fold in diced rhubarb. Divide evenly between muffin cups and sprinkle with streusel.
Bake 20 to 25 minutes until golden brown or toothpick inserted in the center comes out clean.