Preheat your oven to 425F.
Place the flour, milk, eggs, garlic powder and salt in a blender. Blend for 10 seconds, scraping down the sides if necessary and then blend for another 10 seconds.
Allow the batter to rest in the blender for 20 minutes.
Place a 9- or 10- inch oven safe skillet on the middle rack of the oven and remove any racks above it.
Using oven mitts, when hot, remove the skillet from the oven and place it on top of the stove. Add the 3 Tbsps butter and allow it to melt, swirling to coat the bottom and sides of the pan.
Pour the batter on top of the butter. Make sure it runs evenly across all sides of the pan. Place the skillet in the oven and bake until the Dutch Baby is puffed and golden brown, about 20 minutes (Do not open thee oven allowing heat to escape. This will deflate your pancake.)
In the meantime, add remaining 1 tsp butter to a medium sized pan over medium heat. Add the leeks and asparagus. Season with salt and pepper. Saute vegetables until tender, about 5 minutes. Remove from heat and set aside.
Add vegetables to Dutch Baby and sprinkle with parmesan cheese. Serve immediately.