Place nonstick popover pan on lowest rack in oven and preheat oven to 450 degrees (with pan in it).
In the bowl of a stand mixer fitted with the whisk attachment or a large mixing bowl and hand whisk, combine eggs, milk and vanilla. Whisk until frothy, approximately 1 minute.
Add flour, salt, and 1/4 cup sugar to egg mixture.
Whisk flour and salt into egg mixture just until batter is the consistency of heavy cream with some small lumps remaining. There will be air bubbles.
In a small bowl, toss blackberries with 2 tablespoons sugar. Gently fold blackberries and any loose sugar into the batter.
Remove popover pan from oven and coat with cooking spray.
Fill popover cups three-quarters full with batter. Bake 20 minutes, then reduce oven temperature to 350 degrees. Continue to bake until golden brown and dry to the touch, about 20 minutes more.
Remove from pan immediately and poke a small hole in the bottoms to release steam.
Brush tops of baked popovers with melted butter, then roll in remaining sugar.
Serve immediately. Popovers are best fresh and hot.
If you've tried this recipe, come back to let us know how you liked them!