In a blender, blend the milk, eggs and vanilla (if using). Add the flour and salt. Blend, just until the flour is no longer visible. You may need to scrape down the sides of the mixer to make sure all the flour is fully absorbed. Keep the cover on and allow the mixture to rest for 30 minutes.
Cheese Filling
In a large bowl, mash the cheeses together with a fork until well combined. Using the fork, mix in the yolk, melted butter, sugar and salt.
Blueberry Sauce
Measure out ½ cup of the blueberries and set aside. Add the rest of the blueberries and all of the remaining ingredients to a large saucepan. Cook the mixture over medium-low heat, stirring occassionally. Once the sugar is all dissolved, increase the heat to medium-high. Boil the mixture for a few minutes, stirring occasionally. Remove the saucepan from the heat and stir in the remaining ½ cup of blueberries.
Shape and Cook
With medium heat, melt ½ Tablespoon of butter in a skillet or frying pan with an 8-inch diameter cooking surface. When the butter is bubbling, pour approximately ¼ to ⅓ cup of batter into pan. Tilt the pan to spread the batter evenly throughout the bottom surface. Cook until the tops seem dry and the underside is nicely browned, approximately 1-2 minutes. Place the cooked crepe cooked side up on a plate. Continue cooking until all the batter is used, stacking the crepes on top of each other. If crepes start sticking to the pan, add a little slab more of butter to the pan.
Place a crepe, cooked side up, on your work surface. Pipe or place approximately 2 Tablespoons of cheese filling in the bottom third of the crepe. Fold the crepe up (with the filling on it) so that the filling is covered by the crepe. Fold the sides over to the middle, then fold the top over the rest of the crepe. Continue filling and shaping the rest of the blintzes.
Heat ½ Tablespoon of butter in a frying pan over medium heat. Once bubbling, add 2-3 blintzes at a time. Cook until golden brown, approximately 1-2 minutes, then flip and cook until the other side is golden brown.
Notes
For the blueberry sauce, use fresh or frozen blueberries.The cooked blintzes freeze beautifully. Defrost overnight in the refrigerator. Heat in oven at 350° F for approximately 15-20 minutes.