Pour flour, sugar, salt, milk, eggs, and 3 tablespoons melted butter into a blender.
Puree until mixture is smooth, about 30 seconds. Let batter sit for 15 minutes before you use it.
Heat a 12-inch nonstick skillet on medium heat. Lightly coat with additional butter. Add scant 1/3 cup batter and swirl to completely cover bottom of skillet. Cook until underside of crepe is golden brown, 2-3 minutes.
Loosen edge of crepe with a spatula, then with your fingertips, quickly flip. Cook 1 minute more. Slide crepe out of skillet and repeat with remaining batter.
Spread 1 1/2 tablespoons of nutella over each crepe and add a sprinkling of berries. Fold each crepe into a triangle. Top with a dollop of whipped cream and serve immediately. Garnish with mint if desired.
Notes
The crepe batter can be made in advance and stored in the fridge for up to 6 hours. Be sure to stir it well before you use it.
When making crepes, the trick is to use just enough batter to cover the bottom of the pan and work quickly to make sure the batter spreads evenly before it starts to solidify.