Place a nonstick popover pan on lowest rack in oven and preheat the oven to 450°F (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 the 3 eggs , 1 1/2 cups whole milk and 1/2 teaspoon vanilla extract. Whisk until frothy, approximately 1 minute.
Add the 1 1/2 cups flour, 1 teaspoon coarse salt, and 1/4 cup of the sugar to egg mixture.
Whisk the wet ingredients until the batter is the consistency of heavy cream with some small lumps remaining. There will be air bubbles.
In a small bowl, toss 1 cup blackberries with 2 tablespoons of the sugar. Gently fold the blackberries and any loose sugar into the batter. Transfer the batter to a pourable measuring cup.
Remove the popover pan from oven and coat with Cooking Spray. This is easier than vegetable shortening since the pan is hot!
Fill the popover cups three-quarters full with batter. Bake 12 minutes, then reduce the oven temperature to 350°F. Continue to bake until the tops are golden brown and dry to the touch, about 20 minutes more.
Remove the pan immediately and poke a small hole in the bottoms to release the steam.
Brush the tops of baked popovers with the melted 2 tablespoons unsalted butter, then roll in the remaining sugar.
Serve immediately. Popovers are best fresh and hot.
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