For the tempura, whisk the plain flour, cornflour and fizzy water to make a batter the consistency of single cream, adding the water little by little as necessary.
Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Dip the broccoli florets into the batter. Deep-fry the florets in the oil for 5-6 minutes until golden-brown and crisp, then remove from the pan with a slotted spoon and drain on kitchen paper.
For the dipping sauce, blend all the dipping sauce ingredients in a food processor until smooth.
To serve, place the tempura broccoli onto a serving plate with a separate bowl of the dipping sauce on the side.
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