Crayfish and watercress sandwiches

Ingredients

• 1/3 cup (100g) whole egg mayonnaise
• 1/4 cup finely chopped dill
• 40 gram butter, at room temperature, plus 30 g extra, melted
• 10 slices white sandwich bread, crusts removed
• 500 gram cooked crayfish meat, sliced into 5mm rounds
• 100 gram watercress sprigs, roughly chopped, plus extra sprigs to serve
• 1/3 cup finely snipped chives