Preheat oven to 350˚F. Grease a 9”x12” baking dish with butter.
Place rhubarb and ¼ cup sugar into a mixing bowl and toss together. Set aside and allow mixture to macerate for 15 to 20 minutes. Drain rhubarb, discarding excess liquid.
Place eggs, remaining sugar, flour, milk, butter, and salt into a blender and blender for 1 to 2 minutes or until completely smooth.
Pour mixture into the prepared baking dish and top with macerated rhubarb and raspberries.
Bake clafouti for 30 to 35 minutes or until golden brown and puffed.
Remove rom oven and cool for 5 to 10 minutes. Dust with powdered sugar, cut and serve.