Preheat your oven to 425℉ with the muffin tins inside, to heat them up as well.
In a food processor, blend together the flour, milk, eggs, vanilla extract, and salt until well-mixed and slightly bubbly. It's a thin batter and should only take around a minute.
Let the batter rest for about 10 minutes while your oven preheats. If you haven't, chop your butter into small cubes of about ¼ Tbsp in size. You'll be putting one in each muffin cup so make sure you have enough.
Carefully remove the hot muffin tins from the oven. Drop a piece of butter in each; it will melt quickly.
Fill each muffin cup about halfway full. Return your filled tins to the oven to bake.
Bake for 13 minutes. The dutch babies will puff up while baking, but will deflate when removed from the heat.
Let the dutch babies cool for 1-2 minutes before removing them from the tins. Top with whatever you like and enjoy!