Preheat oven to 175C/350F. Line a baking sheet with parchment paper.
In a bowl, mix the flour, cornstarch, baking powder, salt and water until it is a smooth batter. In another bowl, add the breadcrumbs.
One by one, dip the florets in the batter, then into the breadcrumb mixture until evenly coated. Place on the lined baking sheet. (Try to do this with one hand, but I usually wash my hands a couple of times during the process as my fingers get too coated with breadcrumbs/batter).
Repeat until all florets are coated.
Bake for 35-40 minutes, until crisp, flipping halfway through.
Make the sauce by whisking all ingredients together in a bowl.
Toss the florets in the sauce until they are well coated.