Preheat the oven to 220°C (200°C fan) / Gas mark 7 and position a rack near the top.
Clean and devein the prawns if needed, then pat them dry with kitchen paper.
Place the butter in a microwave-safe heatproof bowl and warm until just melted (about 60 seconds, or melt gently on the stove).
Stir the curry powder, white pepper and optional pinch of cayenne into the melted butter until combined.
Add the lemon juice to the butter mixture, taste and season with salt if needed.
Toss the prawns in the lemon curry butter to coat evenly.
Line a baking sheet with kitchen paper, arrange the prawns in a single layer, and bake at the top of the oven for 10 minutes.
Remove from the oven, sprinkle with chopped coriander, and serve immediately.