Combine butter, shallot, lemon rind and chilli in a saucepan and stir over low heat until butter melts (2-3 minutes). Add prawns, season to taste and stir occasionally until prawns turn just opaque (2-3 minutes). Remove prawns with a slotted spoon and transfer to a shallow 500ml bowl, pressing so prawns are level, then pour over enough butter mixture to cover completely. Refrigerate until chilled and firm (2-3 hours).
Meanwhile, for pickled shallots, combine shallot, sugar and a large pinch of sea salt in a non-reactive bowl and stand until tender (1 hour). Add vinegar, season with freshly ground black pepper.
Serve potted prawns on toasted sourdough with lemon wedges to squeeze over and pickled shallots to the side.