How to Make It
Preheat the oven to 350°. Set the squash, cut side down, on a lightly oiled baking sheet and bake for about 45 minutes, or until tender when pierced. Let cool, then scoop out 2 cups of squash.
Heat the olive oil in a medium skillet. Add the leeks and cook over moderate heat, stirring, until softened, about 5 minutes. Stir in the squash, chicken, 1 tablespoon of the maple syrup and the thyme. Season generously with salt and pepper. Cover and set aside.
In a bowl, whisk together the flour and salt; whisk in the eggs and parsley, then whisk in the milk to form a smooth batter. Add the melted butter and mix thoroughly.
Heat a lightly oiled 8-inch skillet. Pour in 3 tablespoons of batter and quickly tilt the skillet in a circular motion to spread the batter evenly. Cook over moderate heat until the blintz is lightly browned on the bottom, 1 to 2 minutes. Transfer the blintz, cooked side up, to a plate. Repeat to make 11 more blintzes.
Spread about 1/4 cup of the filling along 1 edge of the cooked side of the blintz. Fold in 2 sides of each blintz, then roll it up like a burrito; flatten slightly. Set the rolled blintzes, seam side up, on a lightly oiled baking sheet.
In a large heavy skillet, melt 1 tablespoon of the remaining butter over moderately low heat. Add 6 of the blintzes, seam side up, and cook until browned, about 2 minutes. Turn the blintzes and brown the other side. Transfer to a warmed platter and repeat with the remaining butter and blintzes. Serve the blintzes at once, with maple syrup and dollops of sour cream.