Blend the flour, cornstarch, sugar, salt, milk, melted butter and vanilla in a blender for 20 seconds, until completely smooth. Let the batter rest for 20 minutes, which relaxes the flour and makes the eggless crepes strong enough to roll without tearing.
While the batter rests, beat the cream cheese, ricotta, sugar, cornstarch, lemon zest, vanilla and salt until smooth and creamy. Chill the filling until needed.
Combine the berries, sugar and lemon juice in a small saucepan over medium heat. Simmer for 5-7 minutes, until the berries break down into a jammy sauce, then set aside.
Heat an 8-inch nonstick skillet over medium heat and brush it lightly with butter. Pour in 1/4 cup of batter and swirl the pan to coat the bottom in a thin, even layer.Cook for about 2 minutes, until the top looks dry, the edges lift easily and the underside is lightly golden. Slide the crepe onto a plate golden side up and repeat with the remaining batter, brushing the pan with butter as needed.
Place about 3 tablespoons of filling on the lower third of the golden side of each crepe. Fold the bottom edge over the filling, fold in both sides, then roll it up into a neat rectangle and set it seam side down.
Melt 1 tablespoon of butter in a large skillet over medium heat. Fry the blintzes seam side down in batches for 2-3 minutes per side, until golden and crisp and the filling is warm, adding more butter between batches.
Serve the blintzes warm with the berry sauce spooned over the top and a dollop of sour cream.