Preheat oven to 350F. Butter a standard 12 cup muffin tin.
In a large bowl mix together the flour, baking soda, baking powder and salt.
In a medium bowl whisk together the eggs and egg yolk until fully mixed. Slowly whisk in the sugar, melted butter, milk, cream fraiche and vanilla until well combined.
Pour the butter/sugar mixture into the flour mixture and stir gently with a rubber spatula.
Fold in the blueberries and rhubarb.
Spoon the batter into the muffin tin. Bake for 30 minutes or until golden brown.