step 1 For the nuoc cham, whisk together all of the ingredients with 5 tbsp hot water in a small bowl. Set aside while you make the tempura. step 2 Whisk the cornflour, plain flour, sesame seeds (if using) and a large pinch of salt together. Fill a large, deep pan no more than a third full with the vegetable oil and heat until it reaches 180C or a cube of bread dropped in browns in 20 seconds. step 3 Quickly whisk the soda water into the flour mixture, being careful not to overmix, then dunk in the broccoli using tongs. Carefully lower into the hot oil and cook for 2-3 mins until crisp. Drain on kitchen paper, then serve with the nuoc cham on the side for dipping.