PREP: Preheat the oven to 425 degrees F. Spray a 9x13 pan and generously on the bottom and sides with cooking spray. Cut 6 tablespoons of cold (straight from the fridge) butter into 1 tablespoon pieces. Place the pieces in the prepared pan.
COMBINE: In a high-powered blender, combine 1 cup flour, 1 and 1/3 cups whole milk, 3 large eggs, 1 egg yolk, 1/2 teaspoon salt (reduce if using table salt or if sensitive to salt), 1 teaspoon vanilla, 3 tablespoons of white sugar, and (if desired) 1/4 teaspoon ground cinnamon. Before blending mixture, place the prepared pan (with butter in it) in the fully preheated oven.
BLEND: Blend the batter until smooth. Check the pan in the oven; the butter should be mostly melted. Pull out the pan and swirl it around to coat all the bottom of the pan with butter. Return to the oven (if needed) until 100% melted. As soon as the butter is completely melted, immediately pour the batter from the blender into the pan. As you pour, pour in a figure-8 pattern.
BAKE: Place the pan in the oven and bake for 18-23 minutes or until puffed, lightly browned at edges and peaks, and no longer glossy. (This is right at 20 minutes for me) Remove from the oven.
SERVE: Immediately sprinkle on powdered sugar and cut into slices to serve. Serve with fresh berries and buttermilk syrup, if desired. Other serving options are described in the post. Dutch baby is enjoyed right after being made -- it doesn't store well.
BUTTERMILK SYRUP (optional): In a medium-sized pot (bigger than you think, as the syrup doubles or triples in volume!), add in the loosely measured 1/4 cup of dark or light brown sugar, 3/4 cup white sugar, 1/2 cup butter, 1/8 teaspoon salt, and 1/2 cup buttermilk. Heat over medium heat and stir occasionally until butter is melted, sugar is dissolved and syrup is smooth. Reduce the heat to low and simmer for 6-8 minutes. Remove from heat and immediately stir in 1/2 teaspoon baking soda. Whisk quickly to dissolve and evenly disperse the soda. The volume of the syrup will greatly increase. Stir in vanilla extract. Serve warm over pancakes.