Preheat the oven to 375°F. Spray 12 cup mini-muffin pan with nonstick spray.
To a blender, add the buttermilk and eggs and blend on medium speed. While the blender is running, slowly add the flour, butter and salt. Blend until combined, 1 to 2 minutes.
Heat the muffin pan in the oven just until the cooking spray begins to smoke a bit, 1 to 2 minutes. Remove the pan from the oven and carefully spoon the batter into the cups, filling each just to the rim. Bake until the popovers are puffed and golden- 25 to 30 minutes.