Pat the prawns very dry with paper towels and season with salt and white pepper. Wet prawns steam instead of sear, so this step is not optional.
Heat the oil in your widest pan over high heat until it shimmers. Lay the prawns in a single layer and leave them alone for 1 minute. Flip, cook 30 seconds more, then pull them out just short of done. Work in two batches if they crowd the pan.
Drop the heat to medium-low. Add the butter, then the garlic, and cook for about a minute until fragrant and barely golden. If the garlic browns it turns bitter, so keep it moving.
Stir in the soy sauce, sugar, and chili flakes if using.
Return the prawns and any juices to the pan and toss for 30 seconds, just until coated and cooked through. Off the heat, squeeze in the lemon.
Serve over rice, scallions on top, and spoon every last bit of the garlic butter from the pan over the bowls.