Grease bottom of 9 x 13 casserole dish and line with pancakes, cut side down.Pancakes should be standing up, snuggled next to each other. 

In a large bowl, mix milk, cream, sugar and vanilla together. Pour custard mixture over pancakes. Distribute blueberries evenly over the casserole. Cover and refrigerate overnight. Or at the very least 3 hours. You want the pancakes to soak up the cream mixture. 

Remove casserole from refrigerator and preheat oven to 350*. It's important for the casserole to come to room temperature before going into the oven to prevent the dish from breaking. It should rest at least 30 minutes.

While the oven is preheating, prepare the streusel topping. Combine flour, sugar, cinnamon and salt together first, then add butter last. You can "cut the butter in" with a pastry blender or fork. If you have a small food processor, you can mix this in there as well. 

Distribute streusel evenly over the casserole and bake for 50-60 minutes. Mine browned pretty quickly, so you may want to cover it with foil halfway through baking. 

Allow to firm up at least 15-20 minutes.