Warm butter over medium-high heat until melted, then add softshell crab, stir continuously until just cooked, then pass through china cap using wooden pestle to extract all juice.

Sweat tomato, fennel, shallot, and green onion in butter over medium heat for 5–6 minutes, then add passionfruit juice, crab juice with butter, jinda chili, soybean paste, and light palm sugar, cook over low heat for about 2 hours until thickened and reduced, add fennel tops and kaffir lime leaf in final 20 minutes of cooking.

Remove jinda chili, kaffir leaf, and fennel tops, then blend and adjust seasoning with butter, salt, light palm sugar, and xanthan gum as needed, pass through sieve and cool for service.