Heat oil in a heavy pot or deep fryer until it reaches a temperature of 350°F (180°C). Ensure it does not go above 1 1/2-inches.: The sizzle you hear when the batter meets the oil is a comforting signal that the temperature is right, and the visual cue is gentle bubbling around the edges of a test drop. Maintaining about 350°F ensures the exterior crisps quickly while the interior cooks through without absorbing excess oil. I use a candy or deep fry thermometer clipped to the pot for steady readings, and I never crowd the pot, which would cause the temperature to plunge and make the nuggets greasy. If your oil smokes, lower the heat immediately, and discard if it smells burnt. A common mistake is adding too many spoonfuls at once, which yields uneven browning and soggy centers.
Combine flour, baking powder, salt, and sugar in a medium bowl.: Right away you will notice the dry mix has a uniform pale tone, and the sifted texture helps the liquids incorporate smoothly. Mixing these dry ingredients first evenly distributes the baking powder so each nugget gets the same lift. I whisk them together to remove lumps and to aerate the flour slightly, which contributes to a lighter crumb. If you skip this step and add leavening directly to wet ingredients, you risk uneven rise. Avoid packing the flour into the measuring cup; spoon then level for accurate measurement.
In a separate bowl, whisk the egg, milk, and shortening. Combine the wet mixture with the dry mixture. Fold through the corn.: When you whisk the egg with the milk and melted shortening, the mixture becomes glossy and cohesive, which helps the dry ingredients hydrate evenly. Folding, not beating, keeps the batter tender and prevents overdevelopment of gluten. As you fold in the corn , notice the occasional strand of moisture from the kernels; if the batter seems too loose, a tablespoon more flour can balance it. Be gentle to preserve the whole kernels. A frequent error is over mixing, which makes the texture tough rather than soft.
Drop the nugget batter by spoonfuls into the hot oil. Fry for a minute on both sides until golden.: You will hear an enthusiastic sizzle as each spoonful hits the oil, and after about a minute you can look for a warm golden hue at the edges before flipping. The sound changes from an energetic sizzle to a steadier, lower hiss as the nugget cooks through. Turn them with a slotted spoon or spider to avoid splashing. Frying time can vary with spoon size and oil temperature; aim for a light golden brown rather than deep brown. If the exterior browns too fast while the center is still raw, your oil is likely too hot. One mistake to avoid is leaving them too long, which results in an overly dark and dry nugget.
Remove the nuggets from the oil and place them on a wire rack to drain excess oil before serving.: Placing the nuggets on a wire rack lets air circulate, preserving crispness, while paper towels underneath catch the drips. You should hear little clicks as oil drains and see faint wisps of steam rise from the warm centers. Waiting a minute or two before serving allows the interior to set so they do not collapse when bitten. If you pile them on a plate, the steam will soften the crust, so resist that temptation. A common mistake is skipping the rack and stacking immediately, which leads to limp exteriors.