In a medium bowl, whisk together flour, cornstarch, and baking powder. Stir in 2 1/2 cups ice water, being careful not to overmix. Don't worry if batter is lumpy. It should be the consistency of heavy cream; if it's too thick, add more water, 1 tablespoon at a time.
Heat oven to 175 degrees. Heat oil in a 3-quart saucepan to 375 degrees. Hold 1 herb or flower by the stem, and dip it into batter. Lift it out, and hold it for a moment over bowl to let excess drip off, then drop it in oil. Fry only as many herbs at a time as comfortably fit in the pan without touching. Adjust heat as needed to keep temperature at 375 degrees during entire process. Fry herbs until lightly browned on bottom, 30 to 60 seconds, then flip them with a wire skimmer to brown the other side. Drain on paper towels. Keep warm in oven. Repeat with remaining herbs and batter.
When all the herbs are fried, sprinkle with salt. Transfer to a serving platter lined with paper or leaves. Serve immediately.