In a medium saucepan, melt the butter. Add the onion and cook over moderately low heat until softened, about 4 minutes. In a small bowl, whisk 2 tablespoons of the arrowroot into the coconut milk until smooth. Add the coconut milk to the onion and cook over moderately high heat, whisking, until bubbling and thick, about 2 minutes. Remove from the heat and let cool to room temperature. Stir in the crabmeat and season with salt. Roll rounded tablespoons of the mixture into 40 small balls. Transfer the balls to a parchment-lined baking sheet, cover and cool in fridge until firm, about 1 hour. ( you can place in freezer bit DO NOT FREEZE only about 20 minutes. (They should be firm.