Pour flour, sugar, salt, milk, eggs, and 3 tablespoons of melted butter into a blender.
Puree until mixture is smooth, about 30 seconds. Let batter sit at room temperature for 30 minutes before you use it.
Heat a 10-inch nonstick skillet on medium heat. Lightly coat with additional butter. Add 1/4 cup batter and swirl to completely cover bottom of skillet. Cook until underside of pancake is golden brown, 2-3 minutes.
Loosen edge of pancake with a spatula, then with your fingertips, quickly flip. Cook 1 minute more. Slide pancake out of skillet and repeat with remaining batter.
For the filling
Place the filling ingredients in a bowl, then stir until well combined.
Lay a pancake out on a flat surface. Place 3 tablespoons of the filling onto the pancake, about 1 inch away from the bottom of the pancake.
Fold the bottom edge over the filling. Fold both sides in, then gently roll the blintz all the way up. Repeat the process with remaining pancakes and filling.
Place the blintzes in the freezer for 10-15 minutes or until firm,
Melt the remaining 3 tablespoons of butter in a skillet. Place the blintzes in the skillet in a single layer and cook for 4 minutes per side or until golden brown. You may need to work in batches.
Serve the blintzes with raspberry sauce, powdered sugar and berries. Garnish with mint sprigs if desired.